In excel each date have a julian serial number start in 1900
1st january 1900 is day # 1
at this point you can use math to remake all the standard calendar
like 43466 is 1st january 2019 00:00:00 each day can be subdivide in decimal to know hour min and second.
With this simple mathematic you can put hour, minutes and second etc and simplify the database because you only save serial number with maximum of 9 bits precision decimals instead of date.