Confirm the calendar before converting
Historical sources may not label their calendar system because the original audience considered it obvious. A date in an English parish register, a Russian document, or an Ottoman record may need local context before conversion.
If the record comes from a border region, a religious institution, or a period of reform, treat the calendar label as a research question rather than an assumption.
A safer first pass is to write down every clue before calculating: language, location, record creator, stated era, old-style notation, and any neighboring dates. For family-history sources, the workflow in Calendar Conversion for Genealogy Records gives a practical way to preserve those clues beside the converted date.
Watch for old-style year starts
Some historical systems began the legal or customary year on a date other than January 1. That can make dates in January, February, or March appear one year off when modernized.
When you see old-style and new-style notation, preserve the original form in your notes and explain the normalized year separately.
Record your assumptions
A good conversion note says which source calendar was used, which target calendar was produced, and whether the calculation used civil midnight boundaries. For most web conversions this is enough context for another person to reproduce the result.
For legal, religious, academic, or archival conclusions, use converters as aids and verify against specialist references for the exact place and institution involved.