Tool

Bot schedule and time zone planner: cron in UTC, DST checks and next run times

A daily digest at 09:00 for users in another country is a time zone problem first and a cron problem second.

Next occurrences

Store user time zones as IANA names (Europe/Madrid), never as fixed offsets: offsets change with daylight saving time.
Avoid scheduling between 01:00 and 03:00 local time, where DST transitions create skipped or repeated hours.
For per-user schedules, run a frequent UTC tick and select users whose local time matches, instead of one cron per user.