Merge Data by time stamps

Dear all,
I have some device measurements and the time stamps I get from it  
have the below format:

MyStruct$TimeStamps[1,]

[1] 2011.000   10.000    6.000   16.000   23.000   30.539

I can convert them easily with ISOdate() to a number and do the  
calculations I need.

One of my problems is that I want to gather my measurements to piles  
of duration (let's say) 5 minutes.
Afterwards I will apply a function to these piles.
As the device is not super-precise please find below the time needed  
for one operation to complete (in seconds)
.....

1.10
1.90
1.34
1.23
1.56
1.22
1.34
....

as you understand I can not say that 5 minutes measurements are  
specific to X consecutive measurements but differ. How I can ask  
from R to do the summation and whenever there is a 5 minute data set  
to split it so to apply it into a function?

I would like to thank you in advance for your help