Preserve the original date first
Genealogy notes are strongest when they keep the date exactly as the source wrote it. That original form may contain old-style year numbering, a local month name, a religious calendar date, or an abbreviation that later researchers need to inspect.
Add the converted date as a normalized working value, not as a replacement for the source. This keeps your tree searchable while preserving evidence.
A practical entry can include both forms in one note: the transcription from the record, the converter result, and the reason you chose that calendar. Start with the broader checks in Avoiding Mistakes in Historical Date Conversion, then use the relevant converter for the source calendar.
Check place and record office
Calendar reform did not happen everywhere on the same day. A parish, synagogue, civil registry, or colonial office may have followed a rule that differs from a neighboring jurisdiction.
When a record falls near a reform period, treat the location and institution as part of the date. The same written day can need different interpretation depending on who created the record.
Write a reproducible conversion note
A concise note should name the original calendar if known, the target calendar, and the reason for the interpretation. For example, say whether you converted an Old Style Julian date to a modern Gregorian date or simply expanded an ordinal date code.
If the evidence is uncertain, mark the normalized date as tentative. Conversion tools can support the arithmetic, but source interpretation still belongs in the research note.