From The Document Foundation Wiki
How to insert a variable date into a cell
Insert a function into the cell, following these instructions:
- Select the desired cell.
- Go to ▸ .
- In the Categories list, choose Date and time.
- Click on TODAY to get the current date or NOW if you also want to write the hour.
- Click on Next.
- Click on OK.
Since version 3.6 of LibreOffice you can also insert the date as a field:
- The cell must be in the Edit in place state (double click in the cell)
- Right click, choose ▸
If you do not want the date to be updated, use the keyboard as follows:
- Ctrl+; to insert the date.
- Ctrl+⇧ Shift+; to insert the time.