How to count the days until a date
"How many days until…" sounds like subtraction, but a wrong convention gives you an answer off by one. Here is how to get it right every time.
Count calendar days, not 24-hour blocks
The convention for "days until" is calendar days: the number of midnights between today and the target. If today is Thursday and the target is Saturday, the answer is 2 — even though the clock time in between might be 47 or 49 hours.
The day you are standing on counts as zero. That is why "how many days until Saturday" asked on a Saturday returns 0: it is today. Ask it on Sunday and the answer is 6, not 7, because the coming Saturday is six midnights away.
Turning days into weeks and hours
To express the same gap in weeks, divide by 7 and keep the remainder: 100 days is 14 weeks and 2 days. To get hours, multiply by 24 — but remember that daylight saving shifts can make the real elapsed time one hour more or less.
Working days are a different calculation again: skip Saturdays and Sundays, and any holidays your region observes. A 30-calendar-day stretch is usually about 21 or 22 working days.
Why two websites can disagree by one day
Every "days until" answer is anchored to midnight in some time zone. This page counts midnights in your own time zone, in your browser — which is the answer that matches your calendar.
A site that counts in UTC can be a day ahead or behind you around the date line, or late in the evening. If a countdown looks off by exactly one day, that is almost always the reason.