Calendar calculations you can inspect
Calculate Your Age provides free browser-based tools for ages, date intervals and birthday planning. The homepage calculates age in completed years, months and days. Supporting tools compare birth dates, check age at an event, infer birth-date ranges and count calendar days. A separate table lookup shows SSA full retirement age by birth year.
The general tools use the Gregorian calendar. Date differences follow the U.S. Naval Observatory's continuous day-number conversion. Years use completed anniversaries; months are counted from the completed-year anniversary, with missing days clamped to month end. February 29 anniversaries use February 28 in non-leap years. These are stated calculator conventions. Legal and benefit rules can differ.
Sources and verification
Official source links appear with the tools. NIST supports the fixed 24-hour conversion used for date-based hours. The SSA retirement page quotes its official table. Because SSA blocked live requests during the build, the initial table was verified against an archived SSA page from September 2025. The live SSA page remains the source link for current guidance.
The SSA data file records the verification date and verification URL. Its updater validates the official rows and January 1 note before replacing the local file. A failure leaves the last good file available. Calendar arithmetic does not need a changing data feed.
Privacy and limits
Calculator input stays in the browser. The tools do not request names or contact information and do not save your dates in browser storage. See the privacy page for tracking and potential advertising cookies, and the disclaimer for the limits of educational results.
Sources
- U.S. Naval Observatory: Gregorian leap-year rule
- U.S. Naval Observatory: Gregorian dates and continuous day counts
- NIST: the 24-hour day
- NIST: seven-day week terminology
- Social Security Administration: Normal Retirement Age table
Data as of 2026-10-05. SSA table verified using its September 30, 2025 archived page because the live host blocked this server. Confirm current rules with SSA before making a benefit decision.