I have a dataframe as follows:

Date		time		value
20110620	11:18:00	7
20110620	11:39:00	9
20110621	11:41:00	8
20110621	11:40:00	6
20110622	14:05:00	8
20110622	14:06:00	6

For every date, I want to extract the row that has the greatest time.
Therefore, ending up like:
20110620	11:39:00	9
20110621	11:41:00	8
20110622	14:07:00	6

I am using for loops (for every date, find largest time value) to do
this determination, but I am sure there is a simpler way in "R". Has
anyone run into this before?
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