The most common calendars today have 365 days in a year. But many calendars in the past had 360 days or something else. If I wanted to find out the exact day something happened on an old calendar, how could I do that. Wouldn’t the missing five days accumulate and something that was on September 6, 2022 in 365 day calendar appear as September 1, in a 360 day calendar?
We use conversion tables.
For example, the Japanese used the most typical alternative to our solar calendar: a lunar calendar, with twelve months alternating 30 and 29 days. This required the insertion of intercalary months every couple years to counteract the inevitable derivation from the change of seasons. They also used era names, which changed every couple years (basically every time something major happened, be that the installation of a new emperor or the usual disasters like famines, floods, large earthquakes, etc.), so there is no steady accruing number indicating which year it is, which makes things quite annoying to convert to our system.
But since it was universally used, kind people created conversation tables. And other kind people made a database out of it. Similar tables are bound to exist for other calendars.
To be fair, this handy little app, as well as the corresponding tables, only go one way (from the historically used to our contemporary calendar), but why would you ever need to convert something the other way around?