Get list of possible relative date entries

Relative dates can either be a standalone keyword, or they can be a keyword with a duration separated by a semicolon ;.

Examples without duration:

• "value": "yesterday"

• "value": "years_prior" (In this case duration is 0 by default)

Examples with duration:

• "value": "days_later;42"

• "value": "years_prior;1"