Troubles formatting calculated date PHP/MySql

Hi

I have a calculated field - datestamp(insert_time) + minutes (valid_till)
with the sql:

SELECT ...., ....., DATE_ADD(insert_time, INTERVAL valid_till MINUTE) AS
expire_time
etc.

I cannot figure out how to format the expire_time as "H:i" - since the
expire_time field is not a column, DATE_FORMAT(.....) doesn't work.

Also I failed to figure out in PHP too :-)

Any ideas?

Tom