Date Difference
Days, weeks and year-month-day gaps between two dates, plus adding or removing days and months.
What is it
Given two dates, three useful answers: the raw day count (and weeks), and a year-month-day combination that reads naturally — like "2 years, 5 months, 12 days". The day count is computed by subtracting the two timestamps, which handles leap years and month lengths automatically; the year-month-day form borrows days from the previous month, exactly the way people calculate age. Below, you can also add or remove N days or months from a start date.
Notes & fun facts
Why does "exactly 30 years apart" never equal 10957 days? Because leap days sneak in between — that is the whole reason the year-month-day way of expressing age feels natural, while the raw day count is the number computers actually use.
Origin & history
Day-counting between dates became a business necessity in medieval Europe: merchants carried perpetual calendars and tables of "days elapsed" to compute interest and contracts. The modern difference algorithms we use in software trace to the Julian-day-number system formalized by Joseph Scaliger in 1583 — a continuous day count since 4713 BC that makes subtraction trivial.