Sources and Reference Data
External rules and data are included only with a named source, supported scope, and review date. A citation supports a method; it does not turn an estimate into a personal fact.
How sources are selected
Primary standards, government agencies, professional organizations, and original technical documentation are preferred over summaries. A source must directly support the rule or dataset for which it is cited. Search popularity, affiliate value, and convenient wording are not evidence-quality criteria.
What a source record tells you
Each record names the publisher, link, supported scope, last review date, next planned review, and a short limitation note. The applies-to field prevents a reference from being stretched beyond the tools or claims it actually supports.
Calendar and accessibility standards
Temporal and ISO documentation guide date-only, duration, timezone, serialization, and week-number semantics. WCAG 2.2 is the accessibility target for structure, keyboard operation, reflow, focus, contrast, status messages, and equivalent text information.
Health and family references
American College of Obstetricians and Gynecologists guidance supplies pregnancy dating context and trimester boundaries. Government and education sources demonstrate why school requirements vary by location. These pages remain planning aids and do not diagnose, guarantee, or issue an official eligibility decision.
Pet, calendar, and astronomy data
AAHA, AAFP, and AVMA material supports broad pet life-stage context and non-linear educational age comparisons. Hong Kong Observatory tables provide Chinese New Year boundaries for the supported range. NASA orbital periods support educational planetary-age conversions. The limits of each dataset are carried into the result.
Editorial and search guidance
Google Search Central documentation is used as publishing-process guidance: content should be helpful, reliable, people-first, and free from scaled content abuse. It does not certify, endorse, or rank this site, and compliance with guidance never guarantees indexing or search placement.
Review and replacement process
Time-sensitive sources receive shorter review windows than stable standards. If an official source moves, changes scope, becomes outdated, or conflicts with a newer authority, the affected tool is reviewed before the record is replaced. When current official information cannot be verified, the page should disclose that gap instead of inventing certainty.
Reference register
Publishing-process guidance for original utility, trust, authorship, production disclosure, and page experience; not a ranking guarantee.
- Supports
- editorial-policy, content review
- Reviewed
- Review due
Reference for technical eligibility, spam policies, and core publishing practices; inclusion in Search is never guaranteed.
- Supports
- editorial-policy, technical indexability
- Reviewed
- Review due
Used to prohibit scaled, unoriginal content and misleading functionality regardless of how the content was produced.
- Supports
- editorial-policy, ai-usage-policy
- Reviewed
- Review due
Accessibility target for perceivable, operable, understandable, and robust web content; the target is not presented as a blanket conformance claim.
- Supports
- accessibility, interface review
- Reviewed
- Review due
Host provider disclosure for its own infrastructure and users-of-users processing; separate from calculator values handled inside the browser.
- Supports
- hosting, privacy-policy
- Reviewed
- Review due
Primary reference for PlainDate, duration, and ZonedDateTime semantics.
- Supports
- date-only arithmetic, timezones, durations
- Reviewed
- Review due
Used for ISO week-year labels and unambiguous stored date formats.
- Supports
- ISO week, date serialization
- Reviewed
- Review due
One consistent simplified date-range convention; entertainment only.
- Supports
- zodiac-sign
- Reviewed
- Review due
Provides the Chinese New Year boundaries used by the zodiac lookup.
- Supports
- chinese-zodiac
- Reviewed
- Review due
Committee Opinion No. 700, reaffirmed 2025; used for dating-method context.
- Supports
- pregnancy-due-date
- Reviewed
- Review due
Used for the displayed trimester boundaries and non-diagnostic timeline context.
- Supports
- pregnancy-due-date
- Reviewed
- Review due
Supports the decision not to infer one global school eligibility rule.
- Supports
- school-age
- Reviewed
- Review due
Official England guidance: most children start reception in September after their fourth birthday; local application, deferral, and out-of-year decisions still apply.
- Supports
- school-age
- Reviewed
- Review due
Size-banded educational comparison table; not a clinical measurement.
- Supports
- dog-age
- Reviewed
- Review due
Non-linear educational age comparison and broad life-stage context.
- Supports
- cat-age
- Reviewed
- Review due
Supports rejecting a universal seven-human-years shortcut.
- Supports
- dog-age, cat-age
- Reviewed
- Review due
Mean sidereal orbital periods converted to Earth days with documented significant figures.
- Supports
- age-on-planets
- Reviewed
- Review due