Find out how many days are left until any future date.
A countdown measures the remaining interval between now and a future moment. The arithmetic is simple, but it is only as precise as the two endpoints, and the endpoints are where countdowns go wrong. A target given as a date alone has no time attached, so software commonly assumes midnight at the start of that day — which makes a countdown to an event at 6pm read nearly a day short.
Time zones are the second complication and the more consequential one. An event at 9am in one city is a different absolute moment from 9am in another, so a countdown is only correct if both the target and the current time are resolved to the same reference, usually UTC. Daylight saving adds a further wrinkle: if a transition falls between now and the target, one day in the span is 23 or 25 hours, so a countdown computed in whole days can drift by an hour against one computed in seconds. For anything precise — a launch, an exam, a deadline — specify the target with both a time and a time zone.
Usually because the target was entered as a date with no time, so the calculation assumed midnight at the start of that day. Adding the actual event time fixes it.
A given clock time is a different absolute moment in each zone. Both the target and the current time must be resolved to a common reference, normally UTC, or the countdown will be off by the offset between them.
It can, by an hour. If a transition falls within the span, one day is 23 or 25 hours long, so a count in whole days and a count in seconds will disagree slightly.
It depends what you are counting. A countdown to a deadline at end of day includes it; one to an event at a specific time does not. Specifying a time on the target removes the ambiguity entirely.