How it works
Adding to a date rolls forward through months and years and handles leap days, so 31 January plus one month lands on the right day. Finding the difference counts the whole days between two dates and also expresses the gap in years, months, and days.
Add: result = start ± (years, months, weeks, days)
Difference: total days = |date₂ − date₁|
Difference: total days = |date₂ − date₁|