validity period

[Jyothi rao]

Hi,
I have read about the validity period in kannel user guide and tested it for validityperiod=1 in the
smsc group.
within 1 minute if the mobile is not switched on, the message sending fails and returns.
i gave the value as 30 and checked it, but it did not return within 30 minutes.  I read that
the unit is in minutes.
is it really minutes, if so, why is it not working exactly for 30 minutes.
can anyone give more details on validityperiod

thanks in advance
--
Jyothi Rao