Stata stores dates in numeric form using an elapsed-time format. A date variable is interpreted by Stata as the number of days since 1 January 1960. For example: 0 = 1 January 1960; 1 = 2 January 1960; -1 = 31 December 1959.
Time and date-time variables are also stored numerically, however Stata interprets these as the number of milliseconds since 1 January 1960. For example: 0 = 1 January 1960 at 00:00:00.000; 1 = 1 January 1960 at 00:00:00.001; -1 = 31 December 1960 at 23:59:59.999. A time-only variable is formatted such that 1 January 1960 is not shown.
Often when you are first importing or loading a dataset in Stata you will find your date-time variables are stored as strings. There is a range of Stata commands designed to help you convert these string date-times into the appropriate Stata numeric variable. You can use the following cheat sheet to help you work out which commands you need to use and how to format them.