The programmers guide to timestamps and timezones - PyCon India 2023

Transcript

1. The Programmers guide
   to timestamps and time zones
   

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2. Head of engineering, Logic Soft
   @shrayasr
   Shrayas
   

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3. My Aim
   🎯
   

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4. Foster
   conceptual
   thinking.
   

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5. Assumptions
   TODAYS
   

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6. Assumptions
   Leap seconds
   

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7. Assumptions
   Leap seconds
   

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8. Assumptions
   Leap seconds
   Storage layer - PostgreSQL
   

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9. Assumptions
   Leap seconds
   Storage layer - PostgreSQL
   Testing is left as an exercise
   to the listener
   

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10. Agenda
    

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11. Agenda
    Concepts & the chaos
    My recommendations
    Takeaways
    

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12. Concepts
    

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13. EPOCH
    1
    

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14. Reference Point in the past
    THEN
    NOW
    

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15. January 1st 1970 00:00:00
    

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16. INSTANT
    2
    

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18. 󰏝
    

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19. 👀
    󰑑
    👀
    󰑜
    👀
    󰎲
    󰏝
    

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20. INSTANT
    =
    UNAMBIGUOUS
    

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21. EPOCH
    Number of seconds since epoch
    

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22. LOCAL
    DATE
    TIME
    {
    3
    

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23. 👀
    󰑑
    👀
    󰑜
    👀
    󰎲
    󰏝
    

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24. 👀
    󰑑
    👀
    󰑜
    👀
    󰎲 September 17th
    11:15AM
    September 17th
    4:15PM
    September 17th
    12:15PM
    󰏝
    September 17th
    2:45PM
    

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25. 👀
    󰑑
    👀
    󰑜
    👀
    󰎲 September 17th
    11:15AM
    September 17th
    4:15PM
    September 17th
    12:15PM
    󰏝
    September 17th
    2:45PM
    Wall Time
    

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26. TIME ZONES
    4
    

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28. “Yeah, let’s meet at 3:15PM”
    

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29. A group of places, that observes the same
    time on their watches, for any given
    instant.
    

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30. 󰑑 Africa/Dar_es_Salaam
    󰎲 Europe/Berlin
    󰑜 Asia/Ho_Chi_Minh
    󰏝 Asia/Kolkata
    󰑔 America/New_York
    

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31. View Slide

32. UTC
    5
    Coordinated Universal Time
    

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33. 👀
    󰑑
    👀
    󰑜
    👀
    󰎲 September 17th
    11:15AM
    Europe/Berlin
    September 17th
    4:15PM
    Asia/Ho Chi Minh
    September 17th
    12:15PM
    Africa/Dar es Salaam
    󰏝
    September 17th
    2:45PM
    Asia/Kolkata
    

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34. View Slide

35. EPOCH
    Number of seconds since epoch
    

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36. EPOCH
    1694942100 seconds since epoch
    

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37. 1694942100
    instant
    

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38. January 1st 1970 00:00:00
    +
    1694942100s
    

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39. September 17th 2023 9:15AM
    

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40. Europe/Berlin
    September 17th
    11:15AM
    Asia/Ho Chi Minh
    September 17th
    4:15PM
    Africa/Dar es Salaam
    September 17th
    12:15PM
    Asia/Kolkata
    September 17th
    2:45PM
    󰏝
    󰑑 󰑜
    󰎲
    September 17th 2023 9:15AM
    

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41. September 17th 2023 9:15AM
    Europe/Berlin
    September 17th
    11:15AM (+2)
    Asia/Ho Chi Minh
    September 17th
    4:15PM (+7)
    Africa/Dar es Salaam
    September 17th
    12:15PM (+3)
    Asia/Kolkata
    September 17th
    2:45PM (+05:30)
    󰏝
    󰑑 󰑜
    󰎲
    

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42. September 17th 2023 9:15AM (+0)
    Europe/Berlin
    September 17th
    11:15AM (+2)
    Asia/Ho Chi Minh
    September 17th
    4:15PM (+7)
    Africa/Dar es Salaam
    September 17th
    12:15PM (+3)
    Asia/Kolkata
    September 17th
    2:45PM (+05:30)
    󰏝
    󰑑 󰑜
    󰎲
    

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43. REFERENCE TIME ZONE
    

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44. DST
    Daylight Savings Time
    6
    

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45. CHENNAI
    +05:30
    

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46. Sun Graph @ Chennai, India
    

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47. BERLIN
    +02:00
    

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48. Sun Graph @ Berlin, Germany
    

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49. SUMMER TIME
    

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50. DST
    Spring Forward
    Fall Back
    

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51. Sun Graph @ Berlin, Germany
    

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52. Sun Graph @ Berlin, Germany
    SPRING FORWARD
    

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53. On 26th of March, Just as the local time
    reaches 2:00AM, move the clock forward
    to 3:00AM
    Europe/Berlin
    +02:00 -> +01:00
    

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54. 01:58
    01:59
    03:00
    

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55. 01:58
    01:59
    03:00
    

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56. 02:00
    to?
    02:59
    

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57. 02:00
    to
    02:59
    

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58. 02:00
    to
    02:59
    Imaginary Times
    

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59. Sun Graph @ Berlin, Germany
    FALL BACK
    

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60. On 29th of October, Just as the local time
    reaches 3:00AM, move the clock backward
    to 2:00AM
    Europe/Berlin
    +01:00 -> +02:00
    

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61. 02:58
    02:59
    02:00
    

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62. 02:58
    02:59
    02:00
    

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63. 02:00
    to?
    02:59
    

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64. 02:00
    to
    02:59
    02:00
    to
    02:59
    .- OCCURS TWICE .>
    

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65. 02:00
    to
    02:59
    02:00
    to
    02:59
    .- OCCURS TWICE .>
    

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66. 02:00
    to
    02:59
    02:00
    to
    02:59
    .- OCCURS TWICE .>
    Ambiguous Times
    

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67. THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    

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68. THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    THE CHAOS THE CHAOS THE CHAOS THE CHAOS
    The Chaos
    

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70. NO
    

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77. 2023
    

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78. View Slide

79. ✨TZDB✨
    Time Zone Database
    Paul Eggert
    IANA
    

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80. The tzdb is a collaborative compilation of
    information about the world's time zones,
    primarily intended for use with computer
    programs and operating systems
    

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81. ✨
    TZDB
    ✨
    YOUR
    AWESOME
    APP
    

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82. One more thing …
    

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83. ✨
    TZDB
    ✨
    YOUR
    AWESOME
    APP
    

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84. ✨
    TZDB
    ✨
    YOUR
    AWESOME
    APP
    

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85. Decision
    ✨
    TZDB
    ✨
    Windows
    OSX
    Linux
    iOS
    Android
    Languages
    Databases
    Frameworks
    YOUR
    AWESOME
    APP
    

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86. View Slide

87. View Slide

88. Recommendations
    

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89. Case 1
    

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90. Alice
    Bob
    󰏝 󰎲
    Heyy, what’s up? How are you?
    

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91. Alice
    Bob
    󰏝
    󰎲
    Heyy, what’s up? How are you?
    󰐵 SERVER
    

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92. 󰏝
    󰎲
    󰐵 SERVER
    󰑡
    󰏶
    

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93. The server’s operating
    system should be on the
    UTC time zone
    ⭐ RECOMMENDATION 1 ⭐
    

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94. 󰏝
    Bob Heyy, what’s up? How are you?
    At 2:45PM
    

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95. Heyy, what’s up? How are you? 󰏝
    Bob
    󰐵 SERVER
    Message
    

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96. Store timestamps of
    events in the past in UTC
    ⭐ RECOMMENDATION 2 ⭐
    

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97. create table messages
    (
    ...
    message text,
    sent_at timestamp with time zone,
    ...
    );
    

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98. ..> datetime.now(timezone.utc)
    datetime.datetime(2023, 9, 23, 11, 27, 7,
    725900, tzinfo = datetime.timezone.utc)
    

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99. Heyy, what’s up? How are you? 󰏝
    Bob
    󰐵 SERVER
    Message
    

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100. Heyy, what’s up? How are you? 󰏝
     Bob
     󰐵 SERVER
     Message ACK
     

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101. Server communicates
     only in UTC
     ⭐ RECOMMENDATION 3 ⭐
     

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102. 󰏝
     Bob Heyy, what’s up? How are you?
     9:15 AM
     At 2:45PM
     

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103. Client stores UTC &
     converts to local time
     ⭐ RECOMMENDATION 4 ⭐
     

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104. 󰏝
     Bob Heyy, what’s up? How are you?
     2:45PM
     At 2:45PM
     

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105. Heyy, what’s up? How are you?
     2:45PM
     󰏝
     Bob
     Alice 󰎲
     󰐵 SERVER
     Message
     

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106. Heyy, what’s up? How are you?
     2:45PM
     󰏝
     Bob
     Alice 󰎲
     󰐵 SERVER
     Message
     

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107. Alice
     󰐵 SERVER
     󰎲
     Message
     

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108. Server communicates
     only in UTC
     ⭐ RECOMMENDATION 3 ⭐
     

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109. Client stores UTC &
     converts to local time
     ⭐ RECOMMENDATION 4 ⭐
     

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110. Alice
     󰎲
     Heyy, what’s up? How are you?
     11:15AM
     󰐵 SERVER
     

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111. Heyy, what’s up? How are you?
     2:45PM
     󰏝
     Bob
     Alice
     󰎲
     󰐵 SERVER
     Heyy, what’s up? How are you?
     11:15AM
     

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112. Case 2
     

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113. Title: Company All Hands Meeting
     Start: April 28th 2024 - 01:00 PM 󰎺
     

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114. Title: Company All Hands Meeting
     Start: April 28th 2024 - 01:00 PM 󰎺
     󰐵 SERVER
     Send a reminder before
     the meeting
     

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115. Title: Company All Hands Meeting
     Start: April 28th 2024 - 01:00 PM 󰎺
     󰐵 SERVER
     Send a reminder before
     the meeting
     

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116. UTC
     JUST USE
     

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117. NOW
     

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118. NOW
     April 28th 2024, 01:00 PM
     Local Time (Africa/Cairo +3)
     APPOINTMENT
     

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119. NOW
     April 28th 2024, 01:00 PM
     Local Time (Africa/Cairo +3)
     April 28th 2024, 10:00 AM
     UTC +0
     APPOINTMENT
     

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120. NOW
     April 28th 2024, 10:00 AM
     UTC +0
     APPOINTMENT
     

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121. NOW APPOINTMENT
     Egypt to cancel DST and switch
     to the +2 time zone for the
     entire year of 2024
     March 1st, 2024
     

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122. NOW APPOINTMENT
     ✨TZDB✨
     Updated
     

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123. NOW
     April 28th 2024, 10:00 AM
     APPOINTMENT
     UTC +0
     

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124. NOW
     April 28th 2024, 10:00 AM
     APPOINTMENT
     UTC +0
     

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125. NOW
     April 28th 2024, 10:00 AM
     APPOINTMENT
     UTC +0
     April 28th 2024, 12:00 PM
     Local Time (Africa/Cairo +2)
     

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126. April 28th 2024, 01:00 PM
     Local Time (Africa/Cairo +3)
     

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127. April 28th 2024, 01:00 PM
     Local Time (Africa/Cairo +3)
     April 28th 2024, 12:00 PM
     Local Time (Africa/Cairo +2)
     

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128. April 28th 2024, 01:00 PM
     Local Time (Africa/Cairo +3)
     April 28th 2024, 12:00 PM
     Local Time (Africa/Cairo +2)
     

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129. NOW APPOINTMENT
     Egypt to cancel DST and switch
     to the +2 time zone for the
     entire year of 2024
     March 1st, 2024
     

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130. NOW APPOINTMENT
     Egypt to cancel DST and stay in
     the +2 time zone for the entire
     year of 2024
     March 1st, 2024
     RARE?
     

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131. 2013 9
     2014 10
     2015 7
     2016 10
     2017 3
     2018 9
     2019 3
     2020 6
     2021 5
     2022 7
     2023 3
     TZDB
     VERSIONS
     

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132. 2013 9
     2014 10
     2015 7
     2016 10
     2017 3
     2018 9
     2019 3
     2020 6
     2021 5
     2022 7
     2023 3
     TZDB
     VERSIONS
     AVERAGE
     6
     PER YEAR
     

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133. For future events, store
     the wall time with the
     timezone
     ⭐ RECOMMENDATION 5 ⭐
     

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134. create table appointments
     (
     ...
     title text,
     scheduled_at_local timestamp without time zone,
     scheduled_at_timezone text,
     ...
     );
     

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135. create table appointments
     (
     ...
     title text,
     scheduled_at_local timestamp without time zone,
     scheduled_at_timezone text,
     scheduled_at_utc timestamp with time zone,
     ...
     );
     

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136. Every minute, check scheduled_at_utc
     Send notification
     Match found
     

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137. Recommendations
     🔁 A RECAP 🔁
     

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138. 1 Server OS sits in UTC time zone
     

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139. 1 Server OS sits in UTC time zone
     2 Server communicates only in UTC
     

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140. 1 Server OS sits in UTC time zone
     2 Server communicates only in UTC
     3 Store past-ish timestamps as UTC
     

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141. 1 Server OS sits in UTC time zone
     2 Server communicates only in UTC
     3 Store past-ish timestamps as UTC
     4 Store future timestamps as wall time
     with time zone
     

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142. 1 Server OS sits in UTC time zone
     2 Server communicates only in UTC
     3 Store past-ish timestamps as UTC
     4 Store future timestamps as wall time
     with time zone
     5 Client stores UTC and converts to
     local time
     

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143. View Slide

144. Takeaways
     

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145. 1 Think about
     your domain
     1
     

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146. 1. Dates only?
     2. Times only?
     3. Both dates & times?
     4. Microsecond precision?
     5. Customers only in one time zone?
     

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147. 6. Limits on future datetimes?
     7. Any customers from volatile time zones?
     8. Current libraries in the stack
     9. Channel for customer intimation
     

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148. 2
     2Don’t guess.
     Just don’t.
     

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149. 3
     3UTC is good …
     till its not
     

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150. 4Respect the
     TZDB
     4
     

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151. Fin
     @shrayasr
     

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