A test checking that a scheduled reminder appeared on the right day passed every time I ran it on my machine and failed every time it ran in CI, with nothing about the code or the test itself different between the two runs. The only thing that was different was the timezone the two machines were running in.
My machine was set to Eastern time, during the part of the year when Eastern runs four hours behind UTC. The CI runner ran in UTC. The test scheduled a reminder for 11 PM on a given day and checked that it showed up under that same calendar date. At 11 PM Eastern, the date comparison ran fine, because the reminder and the check were both being read in the same local time. Four hours ahead in UTC, 11 PM Eastern is already 3 AM the next day, so the reminder that should have shown up "today" was landing on tomorrow instead, and the test correctly failed against a real bug in how the date was being compared.
The bug lived in the feature: the reminder was meant to fire at one specific scheduled instant, not sometime across the user's local calendar day, so comparing it against "today" only makes sense once both sides are read in the same timezone. Comparing a reminder's timestamp against today without ever converting both to a common timezone first happens to work by accident whenever the two machines involved share the same offset from UTC, and stops working the moment they don't.
