The type of the date
| value | description |
|---|---|
| DAY | A precise day |
| DAYS | An interval between tow days |
| MONTH_IN_YEAR | A month in a year |
| YEAR | A year |
| TO_YEAR | It's only known that the date is before a certain year |
| FROM_YEAR | Don't use, you always can use today as an end date |
| YEARS | An interval between tow years |