[R] time

[solares@unsl.edu.ar]

Hi, i need select data from data frame, for example:
x is a data frame with values in jump to 5 seconds
12:10:00 51 //one minute
12:10:05 63
12:10:10 75
12:10:15 88
..
12:10:59 45
12:11:00 46 //another minute
12:11:05 11
..
Have a command what to select the data minute to  minute, for example
in the example above i need the maximun values minute to  minute
1° minute-->88
2° minute--> 46
Thanks Ruben