irclog: Fix date range calculate bug

When calculate a time range for one day, truncate need to be done on
some time, however the function Truncate in golang is only truncating
time in UTC, for anthor time zone, algin and recovery are needed, to get
the right answer of the start time of one day.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>