If you enter =NOW() in your date field Google Docs will return the current date and time into that cell.
This won't automatically update continuously, but each time you edit your spreadsheet the current time will update. So if you have a timeline entry such as 1980-present, present will be reasonably current so long as you revisit the spreadsheet once in a while.
If you enter =NOW() in your date field Google Docs will return the current date and time into that cell. This won't automatically update continuously, but each time you edit your spreadsheet the current time will update. So if you have a timeline entry such as 1980-present, present will be reasonably current so long as you revisit the spreadsheet once in a while.