Two kinds of deadline
“Submissions close 23:59 Pacific Time” is one instant worldwide. “Offer ends at midnight” often means each person’s own midnight, which arrives at different instants around the world. The countdown supports both and shows which instant it is counting to.
The countdown reads this device’s clock every second. If the clock is wrong the countdown is wrong by the same amount. It does not send reminders or alarms — download the .ics to add the deadline to your calendar.