IconVault's Time Duration Calculator gives you the exact time between any two dates and times, broken into days, hours, minutes and seconds plus total hours, minutes and seconds. A second mode adds or subtracts a duration (days, hours, minutes) from any date, so you can answer questions like what date is 90 days from now or what time was 36 hours ago.

Everything computes instantly in your browser with no signup and nothing sent anywhere, and it is completely free. The honest note: calculations use your device's local calendar math, so results across daylight saving changes reflect your local timezone rules, double-check if you need legal or payroll precision.