Syntax:
TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)

Returns datetime_expr2 − datetime_expr1, where datetime_expr1 and
datetime_expr2 are date or datetime expressions. One expression may be
a date and the other a datetime; a date value is treated as a datetime
having the time part '00:00:00' where necessary. The unit for the
result (an integer) is given by the unit argument. The legal values for
unit are the same as those listed in the description of the
TIMESTAMPADD() function.

This function returns NULL if datetime_expr1 or datetime_expr2 is NULL.

URL: https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html

