Days between two dates
Pick a start and an end date. The answer counts the days from one to the other, so from Monday to Wednesday is 2 days. Turn on Count the end date too when both days should count, as for an event running Monday to Wednesday (3 days). Weekdays only leaves out Saturdays and Sundays; public holidays differ by country and are not removed.
Adding months and years
A month is added on the calendar, not as 30 days: 15 January plus one month is 15 February. When the day does not exist in the new month, the last day is used instead, so 31 January plus one month is 28 February (29 in a leap year).
Working days
With Days are working days on, the days are counted on weekdays only, the way a deadline of "10 working days" is usually worked out. Weekends are skipped, and an answer that would land on a weekend moves past it.
Questions
How many days until a date?
Leave the start date as today and put the date in End date. The start date fills in with today when the page opens.
Does it handle leap years?
Yes. Dates are counted on the real calendar, so 29 February is there in leap years and not in others.
What about time zones?
Dates are counted as whole calendar days, so time zones and clock changes make no difference.