The original number corresponds to all data in the Date and Time formats. Its starting point for all dates is December 30, 1899, whose original number is 0. For example, the original number for the date of January 1, 1900 is 2.
The original number for data in the Date format will always be integer. To see this number, change the cell format to General or Number.
The original number for data in the Time format will always be a number from the 0.0 to 0.999988426 range. To see this number, change the cell format to Number.
The original numbers are required in calculations. Otherwise cells with dates would be considered as text cells and could not act as arguments in formulas and functions.