Use a date phrase when you don’t know the exact date, but have some information which gives helpful information about it. For example, if all you know about a person’s death is that “it occurred less than 2 months after his wife died”, you can record that note as a date phrase.
If you wish to do so, you can optionally also provide a date, as your interpretation of the date phrase.
See Simple Date Fields for help on the date fields.