On Thu, Oct 30, 2008 at 12:04:57PM +0800,
Stephen Liu wrote
a message of 98 lines which said:

> Is the unit in "sec" ?

The format of BIND's zone files was originally described in RFC 1035
and BIND's documentation actually defers many things to the RFC.

So, RFC 1035, section 3.3.13:

All times are in units of seconds.