Link: to a Republican Date Converter
The Republican calendar was used for the period from 1793 to 1806, but Year I of the Republican calendar officially began on 1 January.er Vendémiaire, 22 September 1792.
Le sextile day (sixth epagomenal day) was given at the end of Republican years preceding years in multiples of four. As a result, on 1er Vendémiaire de l'an IV does not correspond to 22 September but to 23 September 1795. Between 23 September 1795 and 29 February 1796 (10 ventôse de l'an IV), the correspondence is therefore shifted by one day.
The year 1800 was not a leap year (1800 is not a multiple of 400), whereas the year VIII was. From 1er March 1800 (10 Ventôse de l'an VIII), so there is a further shift of one day between the two calendars.
To convert dates to one calendar or the other, you can calculate them manually or use tools on the Internet. Some genealogy software packages include a date conversion tool.
Sources :
https://fr.wikipedia.org/wiki/Concordance_des_dates_des_calendriers_républicain_et_grégorien
Photo credit :
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