It's tedious to convert between calendars and this would help at clearing up confusion and superfluous comments on a date.
The discussion at https://community.familysearch.org/en/discussion/83922/use-of-old-style-julian-versus-new-style-gregorian-dates might be of interest to you.
I have never felt the need to convert dates, I just record them to reflect the date as closely as possible to how it was originally written - i.e. how the date was the accepted one (in a particular country) at that time.
Perhaps other genealogists and websites do record things differently, but I have never knowingly encountered these practices in my thirty-five years of working with genealogical data.
There is a related discussion on the FamilySearch research wiki talk page https://www.familysearch.org/wiki/en/Talk:Julian_and_Gregorian_Calendars about the page https://www.familysearch.org/wiki/en/Julian_and_Gregorian_Calendars
Bottom line for me is that FamilyTree should follow historical records. I no longer even attempt to do these calendar conversions. It gets tricky when the month is rendered numerically, not by name, but such records are rare.
"I just record them to reflect the date as closely as possible to how it was originally written - i.e. how the date was the accepted one (in a particular country) at that time."
That doesn't work in edge cases. My great-grandfather's birthdate is written in the Julian calendar in his birth certificate and metrical book, but is in the Gregorian calendar in his marriage certificate and everything after.
Dominik, see Gordon's comment on the thread that Paul referenced for instructions on entering dates to display on FS just about any conceivable way.
The genealogical standard for places is to "use the name at the time of the event". If a place had more than one name at the same time (for example in different languages), then the "rule" (-of-thumb) is to use whichever one is in the record itself (or matches it most closely). The same principle applies to dates: you should enter a display date in the Julian calendar for your great-grandfather's birth, but display the Gregorian date for his marriage and death.
Unfortunately, using a different calendar (or a different set of names for the months, like a lot of people did in the spirit of nationalism in the mid-1800s) will require a bit of research and calculation. FS's standardization routine doesn't do any conversion between calendars, and only knows the standard names of the months in whatever language your interface is set to. I'm not sure it makes sense to implement a date converter-and-calculator in FS; it's a genealogy-adjacent task, not genealogy itself. I'd much rather look it up on a specialized website than expect a site with a different focus to get it right.
On a global scale the reconciliation of dates according to different calendars is extremely complex. Hence the standard practice to record dates as written in the historical record. Even so, there are edge cases where this is difficult. Especially difficult are old calendar dates written with numerical months. The existence of edge cases does not invalidate the standard practice.