Do Cosmos reminders handle daylight saving time?

Yes. Each reminder keeps its time zone, and repeating reminders keep the same wall-clock time. A daily 08:00 reminder still comes at 08:00 local time after the clocks change.

Example#

You set "every day at 08:00" in Africa/Cairo. When Egypt moves its clocks forward or back, the reminder still arrives at 08:00 on your clock, not an hour early or late.

One-off reminders are fixed to the exact local date and time you chose. If you travel to another time zone, see the travel article: existing reminders keep the zone they were set in.