[R] A data selection problem, suggestions highly appreciated

[Yong Wang]

Hi, All
I get following question:
A data format like following:

[Day time  x y]

Jan1 18:56:24 x1 y1
Jan1 18:56:25 x2 y2
Jan1 18:56:27 x3 y3
Jan1 18:56:28 x4 y4
Jan1 18:56:31 x5 y5
.....................
.....................

what I wanna do is to partion the time interval by unit of 5 seconds.
and pick x,y corresponding to the last time within that interval. for the 
example above,
suppose 18:56:24 is the starting time, the time included in the interval 
will be up to 18:56:28, therefore I want to get x4 and y4, and store them 
somewhere else. 
you might know this is the so called high frequency data. can you let me 
know how to do this partion and pick in R or Splus. 
suggestions are really and hihgly appreciated. 

thank you very much.
best regards
yong