I want to make an update query of the following select query(with count), but don't know how to do it (it's been a while):

SELECT T1.Date, T1.Time, (SELECT COUNT(*) FROM mytable T2 WHERE T1.Date=T2.Date AND T2.Time<=T1.Time) AS Daycount
FROM mytable AS T1
ORDER BY T1.Date, T1.Time;

This query numbers registered times / day, which i need as columnnumber in a crosstable.
All fields are present in mytable.