Use a single source of truth
State the deadline in one reference zone and, when useful, include UTC. For example, submissions close at 17:00 Singapore time, UTC+08:00.
Avoid accepting entries based on each participant's local midnight unless that is truly intended and clearly written.
Watch daylight saving changes
A deadline planned weeks ahead may shift relative to another country if one place changes daylight saving time and the other does not.
Check the offset again shortly before the deadline, especially for March, April, October, and November dates.
Leave a submission buffer
Payment gateways, file uploads, verification emails, and travel forms can fail near the cutoff. Encourage users to submit early.
For official legal, academic, or immigration deadlines, confirm the cutoff with the organization that owns the rule.