What do you call someone who studies calendars?

I was looking for a word that describes someone who is an expert in or studies calendars, and I couldn't find anything.

For more context, I am looking for the word you would call someone who knows how to calculate the number of days between a date in the distant past, when different calendars were used, and a date in the modern calendar. I thought historian, but I'm not sure that's adequate.

I believe that you are looking for a horologist. (Horology: the art or science of measuring time.)

Wikipedia has this to offer, with a citation to a 1913 Webster's.

Chronometry (from Greek χρόνος "time" and μέτρηση "measurement") is the science of the measurement of time, or timekeeping. It should not to be confused with chronology, the science of locating events in time, which often relies upon it.