Illustrative example. The dates and amounts below are provided for practice.
A month-end birth exposes why calendar months cannot be treated as fixed thirty-day units. This example follows the anchor through a leap February.
Enter and compare
Set Date of Birth to January 31, 2000 and Calculate Age As Of to March 1, 2024. Calculate to obtain 24 years, one month, and one day.
The last completed birthday is January 31, 2024. Adding one month clamps the date to February 29. One elapsed day remains. Change the reference to February 29 and the Days remainder becomes zero.
Preserve the original anchor
The calculator evaluates month offsets from the anniversary anchor. It does not repeatedly move a truncated February date forward and permanently change the monthly day to the 29th.
For a March 31 reference in 2024, two whole months have passed since January 31. That would be lost by a workflow that treated February 29 as the new permanent anchor.
The output describes calendar age. Total Days Lived uses elapsed intervals from birth, and inclusive day-of-life numbering would add one. When comparing results from another method, check both the month-end rule and the reference date before assuming either result is erroneous.