Time Calculator
Find the gap between two moments in days, hours, minutes and seconds, or shift a start moment by any duration.
What is it
Two jobs here: (A) measure how long two moments are apart — enter both date-times and get days, hours, minutes and seconds between them, plus the total in seconds. (B) take a start moment and add or subtract a duration in days, hours, minutes and seconds, then read off the resulting moment. Crossing months and years is handled by the browser date engine, so leap years and 30-vs-31-day months just work.
Notes & fun facts
Computers store time as a single number: milliseconds since 1970-01-01 UTC. Every calendar question you ask is really just subtraction and addition on that counter — all the messy rules about 30-day months and leap years are applied only when the number is formatted back into a date.
Origin & history
Measuring elapsed time between two moments became a daily problem in the 19th century, when railways needed shared timetables. Before that, every town kept its own solar time, so noon in one city was not noon in the next. Modern Unix time — seconds since 1970-01-01 — was designed at Bell Labs in the early 1970s and quietly handles nearly all calendar math today.