[R] Aggegate minutes data to hourly data

[Kuma Raj]

I have a measurement that was taken in 15 minutes or more and want to
aggregate it by hour. How could I do that?

Sample data is found below
date_time concentration
26/11/2013 15:46 529.25
26/11/2013 16:03 1596
26/11/2013 16:23 1027.111
26/11/2013 16:39 1001.9
26/11/2013 16:54 -80.25
26/11/2013 17:12 1064.125
26/11/2013 11:14 7969.7
26/11/2013 11:32 522
26/11/2013 11:58 845.111
26/11/2013 12:12 1166.875
26/11/2013 12:30 473.375
26/11/2013 12:42 466.2
26/11/2013 07:47 4358.833
26/11/2013 08:05 1257.545
26/11/2013 08:24 828.6
26/11/2013 08:45 942
26/11/2013 08:58 758.111
26/11/2013 09:13 832.333
26/11/2013 15:45 1876.909
26/11/2013 16:07 574.25
26/11/2013 16:27 1736.846
26/11/2013 16:43 1024.857
26/11/2013 16:59 858.538
26/11/2013 17:15 912.455
26/11/2013 11:18 2086.143
26/11/2013 11:39 2078.667
26/11/2013 12:03 1619.072
26/11/2013 12:16 1197.583
26/11/2013 12:35 619.308
26/11/2013 12:51 1222.571
26/11/2013 07:49 1357.929
26/11/2013 08:08 1120
26/11/2013 08:29 1381.6
26/11/2013 08:48 1493.429
26/11/2013 09:03 1113.786
26/11/2013 09:18 1217.143