Birth From Death Date Calculator

Genealogical records, obituaries, and gravestones frequently list a death date alongside an age but omit the birth date. Reconstructing the exact date of birth from a death date and a stated age (in years, months, and days) requires calendar-aware subtraction that accounts for unequal month lengths and leap years. A naive subtraction of 365.25 × years days produces errors of up to ±2 days. This tool performs exact calendar arithmetic: it walks backward month-by-month from the death date, respecting each month's true length and February's leap-day rules. It also operates in forward mode, computing the precise lifespan between a known birth date and death date, decomposed into Y years, M months, and D days.

Results include total days lived, day of the week for both dates, Western zodiac sign, Chinese zodiac animal, and the percentage of a statistical average lifespan (72.6 years, WHO 2023 global figure) that was reached. Pro tip: historical records before the Gregorian calendar adoption (1582 in Catholic countries, 1752 in Britain) may use Julian dates. This tool assumes Gregorian throughout. Adjust old-style dates manually before input.

Forward mode computes the exact calendar age between a birth date B and a death date D.

Where Y, M, D are obtained by calendar subtraction: start at B, increment full years until exceeding D, then increment full months, then count remaining days. Total days lived:

where D_ms and B_ms are Unix timestamps in milliseconds. Reverse mode reconstructs birth date from death date and stated age:

Leap year check for any year y:

Percentage of average lifespan:

where 72.6 is the WHO 2023 global average life expectancy. Life path number (numerology): recursively sum all digits of the full birth date (DDMMYYYY) until a single digit 1 - 9 remains (master numbers 11, 22, 33 are preserved).