[R] choose between dates and times

[Kerry Bush]

Dear R-helpers,
  I have the following data:

 y    happenat                 x
5185 (07/22/05 00:05:14)       14
5186 (07/22/05 00:15:14)       14
5187 (07/22/05 00:25:14)       14
5188 (07/22/05 00:35:14)       14
......

I want to choose between 07/25/05 15:30:00 and
07/26/05 12:30:00. Anybody had experience in handling
this kind of data? Is there a simple way to subset by
the variable 'happenat'? Thanks.