Date and Timezone Bug Checklist

Review how this feature stores and displays time. Find the timezone bugs before users in another country do. FEATURE What the time means: [AN EVENT IN A PLACE | A DEADLINE | A LOG LINE | A BIRTHDAY] Who is in which zones: [USER ZONES] Database: [POSTGRES] Display: [BROWSER LOCALE | A ZONE THE USER CHOSE] WALK THROUGH AND WRITE THE DECISION FOR EACH 1) The type in Postgres (timestamptz versus a date with no time) and why the other one is wrong here. 2) The wire format (ISO 8601 with offset) and the bug if we send a string without a zone and let the server assume UTC or local. 3) "End of day" and who decides which day. Inclusive deadlines. 4) A DST transition: an event at 1:30 when the hour repeats or skips. What we store. 5) Birthdays and other zone-free dates, which must not become timestamptz. 6) Tests: a user in Kolkata and a user in New York looking at the same event, and a deadline that is "Friday" in the user's zone. 7) The library we use and the one we will not (hand-rolled offset math). Output a short design note plus the tests. Do not convert everything to UTC and also format it with the server's local zone.