R: which apply function for conditioned iteration?

I'm new to R and I'm wondering which the fastest and most elegant way to do the iteration I'm going to describe. I have a big dataframe formatted like this:

   date_time      value1   value2
01-02-1986 00:00  100.120  200.150
01-02-1986 00:01  100.125  201.120
...
01-02-1986 23:59  100.950  190.130
02-02-1986 00:00  103.110  108.200
...

so there are one date and two numeric columns and the rows go on minute by minute.

For each day I need to take df$value1 at given times (say 10:00, 11:00, 15:00), for each pass ceil it to its first decimal, find out the highest value above this in the next Y minutes and put it in a matrix so that I will end up with something like:

<dd-mm-yy> <highest at time 1> <highest at time 2> <highest at time 3>         
...
<dd-mm-yy> <highest at time 1> <highest at time 2> <highest at time 3>

I have a vector of the HH:MM I need and a subset of df that contains only the pertinent hours (10:00 to 10:59, 11:00 to 11:59 and 15:00 to 15:59) but now I'm stuck on the iteration.

Is there a way to do this without a for/if cycle rolling the whole array?

Thank you for your help.