If you’ve ever used a spreadsheet, you’ve probably used or seen functions for doing date math—in other words, taking one date and adding some number of days or months to it to get a new date, or taking two dates and finding the number days between them. The same thing can be done from the command line using the lowly date command, possibly with a little help from Bash’s arithmetic.

Source: Doing Date Math on the Command Line, Part I | Linux Journal

Source: Doing Date Math on the Command Line – Part II | Linux Journal