I had a CSV file that needed to be processed on a web page. So I had to convert the Excel time to UTC time.
According to this answer on StackOverflow:
The Excel number for a modern date is most easily calculated as the number of days since 12/30/1899 on the Gregorian calendar.
Excel treats the mythical date 01/00/1900 (i.e., 12/31/1899) as corresponding to 0, and incorrectly treats year 1900 as a leap year. So for dates before 03/01/1900, the Excel number is effectively the number of days after 12/31/1899.