Add one month to January 31st using naive date-library math and you can end up on March 3rd — because "February 31st" doesn't exist, so the extra 3 days spill into March. Most date calculators never tell you which behavior they use.
Day counter, whole days between two dates
Switch to "Days between two dates" and enter any two dates — the result counts whole days from the start of the earlier date to the start of the later date (Jan 1 to Jan 2 is 1 day, not 2), plus a years/months/days breakdown and a total-weeks figure.
Add days to a date, or subtract, without the month-end surprise
Switch to "Add/subtract from a date" and enter years, months, weeks, and days to add or remove. When adding a month lands on a date that doesn't exist in the target month, this calculator clamps to the last valid day — Jan 31 + 1 month = Feb 28 (Feb 29 in a leap year) — instead of overflowing into March.
