Because this is ISO 8601 date format, the standard date format that intentionally uses dashes to not confuse 25/01/22, 01/22/25 and 22/01/26 which are the same date in different formats
(that's why it uses dashes for dates, as for why it uses / for interval I have no idea)
(that's why it uses dashes for dates, as for why it uses / for interval I have no idea)