The programmers guide to timestamps and timezones - PyCon India 2023

Transcript

1. The Programmers guide to timestamps and time zones
2. Head of engineering, Logic Soft @shrayasr Shrayas
3. My Aim 🎯
4. Foster conceptual thinking.
5. Assumptions TODAYS
6. Assumptions Leap seconds
7. Assumptions Leap seconds
8. Assumptions Leap seconds Storage layer - PostgreSQL
9. Assumptions Leap seconds Storage layer - PostgreSQL Testing is left as an exercise to the listener
10. Agenda
11. Agenda Concepts & the chaos My recommendations Takeaways
12. Concepts
13. EPOCH 1
14. Reference Point in the past THEN NOW
15. January 1st 1970 00:00:00
16. INSTANT 2
18. 󰏝
19. 👀 󰑑 👀 󰑜 👀 󰎲 󰏝
20. INSTANT = UNAMBIGUOUS
21. EPOCH Number of seconds since epoch
22. LOCAL DATE TIME { 3
23. 👀 󰑑 👀 󰑜 👀 󰎲 󰏝
24. 👀 󰑑 👀 󰑜 👀 󰎲 September 17th 11:15AM September 17th 4:15PM September 17th 12:15PM 󰏝 September 17th 2:45PM
25. 👀 󰑑 👀 󰑜 👀 󰎲 September 17th 11:15AM September 17th 4:15PM September 17th 12:15PM 󰏝 September 17th 2:45PM Wall Time
26. TIME ZONES 4
28. “Yeah, let’s meet at 3:15PM”
29. A group of places, that observes the same time on their watches, for any given instant.
30. 󰑑 Africa/Dar_es_Salaam 󰎲 Europe/Berlin 󰑜 Asia/Ho_Chi_Minh 󰏝 Asia/Kolkata 󰑔 America/New_York
32. UTC 5 Coordinated Universal Time
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
35. EPOCH Number of seconds since epoch
36. EPOCH 1694942100 seconds since epoch
37. 1694942100 instant
38. January 1st 1970 00:00:00 + 1694942100s
39. September 17th 2023 9:15AM
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
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) 󰏝 󰑑 󰑜 󰎲
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) 󰏝 󰑑 󰑜 󰎲
43. REFERENCE TIME ZONE
44. DST Daylight Savings Time 6
45. CHENNAI +05:30
46. Sun Graph @ Chennai, India
47. BERLIN +02:00
48. Sun Graph @ Berlin, Germany
49. SUMMER TIME
50. DST Spring Forward Fall Back
51. Sun Graph @ Berlin, Germany
52. Sun Graph @ Berlin, Germany SPRING FORWARD
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
54. 01:58 01:59 03:00
55. 01:58 01:59 03:00
56. 02:00 to? 02:59
57. 02:00 to 02:59
58. 02:00 to 02:59 Imaginary Times
59. Sun Graph @ Berlin, Germany FALL BACK
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
61. 02:58 02:59 02:00
62. 02:58 02:59 02:00
63. 02:00 to? 02:59
64. 02:00 to 02:59 02:00 to 02:59 .- OCCURS TWICE .>
65. 02:00 to 02:59 02:00 to 02:59 .- OCCURS TWICE .>
66. 02:00 to 02:59 02:00 to 02:59 .- OCCURS TWICE .> Ambiguous Times
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
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
70. NO
79. ✨TZDB✨ Time Zone Database Paul Eggert IANA
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
81. ✨ TZDB ✨ YOUR AWESOME APP
82. One more thing …
83. ✨ TZDB ✨ YOUR AWESOME APP
84. ✨ TZDB ✨ YOUR AWESOME APP
85. Decision ✨ TZDB ✨ Windows OSX Linux iOS Android Languages Databases Frameworks YOUR AWESOME APP
88. Recommendations
89. Case 1
90. Alice Bob 󰏝 󰎲 Heyy, what’s up? How are you?
91. Alice Bob 󰏝 󰎲 Heyy, what’s up? How are you? 󰐵 SERVER
92. 󰏝 󰎲 󰐵 SERVER 󰑡 󰏶
93. The server’s operating system should be on the UTC time zone ⭐ RECOMMENDATION 1 ⭐
94. 󰏝 Bob Heyy, what’s up? How are you? At 2:45PM
95. Heyy, what’s up? How are you? 󰏝 Bob 󰐵 SERVER Message
96. Store timestamps of events in the past in UTC ⭐ RECOMMENDATION 2 ⭐
97. create table messages ( ... message text, sent_at timestamp with time zone, ... );
98. ..> datetime.now(timezone.utc) datetime.datetime(2023, 9, 23, 11, 27, 7, 725900, tzinfo = datetime.timezone.utc)
99. Heyy, what’s up? How are you? 󰏝 Bob 󰐵 SERVER Message
100. Heyy, what’s up? How are you? 󰏝 Bob 󰐵 SERVER Message ACK
101. Server communicates only in UTC ⭐ RECOMMENDATION 3 ⭐
102. 󰏝 Bob Heyy, what’s up? How are you? 9:15 AM At 2:45PM
103. Client stores UTC & converts to local time ⭐ RECOMMENDATION 4 ⭐
104. 󰏝 Bob Heyy, what’s up? How are you? 2:45PM At 2:45PM
105. Heyy, what’s up? How are you? 2:45PM 󰏝 Bob Alice 󰎲 󰐵 SERVER Message
106. Heyy, what’s up? How are you? 2:45PM 󰏝 Bob Alice 󰎲 󰐵 SERVER Message
107. Alice 󰐵 SERVER 󰎲 Message
108. Server communicates only in UTC ⭐ RECOMMENDATION 3 ⭐
109. Client stores UTC & converts to local time ⭐ RECOMMENDATION 4 ⭐
110. Alice 󰎲 Heyy, what’s up? How are you? 11:15AM 󰐵 SERVER
111. Heyy, what’s up? How are you? 2:45PM 󰏝 Bob Alice 󰎲 󰐵 SERVER Heyy, what’s up? How are you? 11:15AM
112. Case 2
113. Title: Company All Hands Meeting Start: April 28th 2024 - 01:00 PM 󰎺
114. Title: Company All Hands Meeting Start: April 28th 2024 - 01:00 PM 󰎺 󰐵 SERVER Send a reminder before the meeting
115. Title: Company All Hands Meeting Start: April 28th 2024 - 01:00 PM 󰎺 󰐵 SERVER Send a reminder before the meeting
116. UTC JUST USE
117. NOW
118. NOW April 28th 2024, 01:00 PM Local Time (Africa/Cairo +3) APPOINTMENT
119. NOW April 28th 2024, 01:00 PM Local Time (Africa/Cairo +3) April 28th 2024, 10:00 AM UTC +0 APPOINTMENT
120. NOW April 28th 2024, 10:00 AM UTC +0 APPOINTMENT
121. NOW APPOINTMENT Egypt to cancel DST and switch to the +2 time zone for the entire year of 2024 March 1st, 2024
122. NOW APPOINTMENT ✨TZDB✨ Updated
123. NOW April 28th 2024, 10:00 AM APPOINTMENT UTC +0
124. NOW April 28th 2024, 10:00 AM APPOINTMENT UTC +0
125. NOW April 28th 2024, 10:00 AM APPOINTMENT UTC +0 April 28th 2024, 12:00 PM Local Time (Africa/Cairo +2)
126. April 28th 2024, 01:00 PM Local Time (Africa/Cairo +3)
127. April 28th 2024, 01:00 PM Local Time (Africa/Cairo +3) April 28th 2024, 12:00 PM Local Time (Africa/Cairo +2)
128. April 28th 2024, 01:00 PM Local Time (Africa/Cairo +3) April 28th 2024, 12:00 PM Local Time (Africa/Cairo +2)
129. NOW APPOINTMENT Egypt to cancel DST and switch to the +2 time zone for the entire year of 2024 March 1st, 2024
130. NOW APPOINTMENT Egypt to cancel DST and stay in the +2 time zone for the entire year of 2024 March 1st, 2024 RARE?
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
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
133. For future events, store the wall time with the timezone ⭐ RECOMMENDATION 5 ⭐
134. create table appointments ( ... title text, scheduled_at_local timestamp without time zone, scheduled_at_timezone text, ... );
135. create table appointments ( ... title text, scheduled_at_local timestamp without time zone, scheduled_at_timezone text, scheduled_at_utc timestamp with time zone, ... );
136. Every minute, check scheduled_at_utc Send notification Match found
137. Recommendations 🔁 A RECAP 🔁
138. 1 Server OS sits in UTC time zone
139. 1 Server OS sits in UTC time zone 2 Server communicates only in UTC
140. 1 Server OS sits in UTC time zone 2 Server communicates only in UTC 3 Store past-ish timestamps as UTC
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
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
144. Takeaways
145. 1 Think about your domain 1
146. 1. Dates only? 2. Times only? 3. Both dates & times? 4. Microsecond precision? 5. Customers only in one time zone?
147. 6. Limits on future datetimes? 7. Any customers from volatile time zones? 8. Current libraries in the stack 9. Channel for customer intimation
148. 2 2Don’t guess. Just don’t.
149. 3 3UTC is good … till its not
150. 4Respect the TZDB 4
151. Fin @shrayasr