Excel Date Serial Number Converter
Convert Excel serial numbers such as 45000 to dates and back, in the 1900 or 1904 date system.
2023-03-15 is serial 45,000
=DATE(2023,3,15)Serial 45000 is 2023-03-15
=TEXT(A2,"yyyy-mm-dd hh:mm")Why Excel shows numbers instead of dates
Excel stores each date as a count of days. In the 1900 date system, used by default on Windows and in current Mac versions, 1 January 1900 is 1 and 1 January 2023 is 44927. A time is the fraction of a day, so 45000.75 is 15 March 2023 at 18:00. If a date cell shows a number like 45000, it has a number format applied: select it and choose a date format from Home > Number.
The 1900 leap year
Excel treats 1900 as a leap year, which it was not, so serial 60 is “29 February 1900”. Microsoft keeps this for compatibility with Lotus 1-2-3. It only affects dates before 1 March 1900.
The 1904 date system
Workbooks created in older Mac versions may use the 1904 date system, where serial 0 is 1 January 1904. The same date is 1,462 lower than in the 1900 system. If dates jump by about four years when you copy between workbooks, the two files use different systems; the setting is under File > Options > Advanced > “Use 1904 date system”.
Range
Excel dates run from 1 January 1900 (serial 1) to 31 December 9999 (serial 2958465).
Sources: Function behaviour: Microsoft Support function reference. Results checked against worked examples, and generated formulas evaluated in the open-source HyperFormula engine. Method & sources