Birthday Calculator
This Birthday & Age Calculator computes age from a provided date of birth to a specified reference date. Default reference values are provided for convenience but may be changed for historical or projection scenarios.
Outputs include age in full years, total months, an approximate count of days, and an approximate countdown to the next birthday. Results are intended for general civil use (ID verification, eligibility checks, personal information) rather than legal or medical certification.
Governance
Record 7d1310f73e59 • Reviewed by Fidamen Standards Committee
Inputs
Results
Age (years)
35
Age (total months)
431
Age (approximate days)
13,114
Days until next birthday (approx.)
30
| Output | Value | Unit |
|---|---|---|
| Age (years) | 35 | years |
| Age (total months) | 431 | months |
| Age (approximate days) | 13,114 | days |
| Days until next birthday (approx.) | 30 | days |
Visualization
Methodology
Age in years is computed using civil calendar arithmetic: the difference in years between reference and birth year, minus one when the reference month/day is earlier than the birth month/day. This aligns with common legal and administrative definitions of age.
Total months equals years times 12 plus the month difference, adjusted down by one month when the reference day is before the birth day.
Approximate days are calculated using a 365-day base plus leap-year corrections (Gregorian rules). Months are approximated as 30 days for summary outputs. For precise day counts use a dedicated date-duration tool that accounts for exact month lengths and calendar transitions.
Accuracy notes: this tool uses civil (Gregorian) rules and integer arithmetic. It does not account for time zones, time-of-day, fractional seconds, or leap-second insertions. For high-integrity applications consult standards and authoritative time sources.
F.A.Q.
Is this calculation exact to the day?
The years and months calculation follows civil calendar rules and is exact for those units. The 'approximate days' and 'days until next birthday' outputs use month/day approximations for summary purposes. For exact day counts use a dedicated date-duration calculation that counts exact month lengths and accounts for calendar transitions.
Does this account for time zones and leap seconds?
No. This tool performs civil-date arithmetic (year, month, day) and does not consider time-of-day, time zones, or leap-second events. For timestamp-level precision consult authoritative time services and NIST guidance.
Which calendar does this use?
This tool assumes the proleptic Gregorian calendar for all years. For historical dates prior to Gregorian adoption or for non-Gregorian calendars, use a calendar-aware conversion tool.
Can I calculate age on a past or future reference date?
Yes. Change the reference date fields to compute age as of any given date, useful for retrospective verification or future eligibility planning.
How should I use these results for legal or medical decisions?
Treat results as informational. For legal, medical, or compliance decisions require authoritative documentation and, where required, certified date calculations or practitioner confirmation.
Sources & citations
- NIST Time and Frequency Services (leap seconds and civil time guidance) — https://www.nist.gov/pml/time-and-frequency-division
- ISO 8601 — Date and time format — https://www.iso.org/iso-8601-date-and-time-format.html
- Gregorian calendar and leap year rules (reference summary) — https://www.timeanddate.com/calendar/gregorian-leap-year.html
- IEEE guidelines on time synchronization for distributed systems — https://standards.ieee.org/
- OSHA guidance and records retention (age-related recordkeeping contexts) — https://www.osha.gov/
Further resources
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
Record ID: 7d1310f73e59What changed (latest)
v1.0.0 • 2025-11-23 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
What changed (latest)
v1.0.0 • 2025-11-23 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Engine
v1.0.0
Data
Baseline (no external datasets)
Content
v1.0.0
UI
v1.0.0
Governance
Last updated: Nov 23, 2025
Reviewed by: Fidamen Standards Committee (Review board)
Credentials: Internal QA
Risk level: low
Reviewer profile (entity)
Fidamen Standards Committee
Review board
Internal QA
Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person
Semantic versioning
- MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
- MINOR: New features or fields that do not change existing outputs for the same inputs.
- PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.
Review protocol
- Verify formulas and unit definitions against primary standards or datasets.
- Run golden-case regression suite and edge-case suite.
- Record reviewer sign-off with credentials and scope.
- Document assumptions, limitations, and jurisdiction applicability.
Assumptions & limitations
- Uses exact unit definitions from the Fidamen conversion library.
- Internal calculations use double precision; display rounding follows the unit's configured decimal places.
- Not a substitute for calibrated instruments in regulated contexts.
- Jurisdiction-specific rules may require official guidance.
Change log
v1.0.0 • 2025-11-23 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: a55ef9a22a47