[R] How to sum and group data by DATE in data frame

[Cren]

Michael Weylandt wrote
> 
> Can you provide a reproducible example? 
> 
Of course, Michael.

Consider the following time series:


11/2/2011 14:30	123.53
11/2/2011 15:00	123.78
11/2/2011 15:30	124.24
11/2/2011 16:00	124.2
11/2/2011 16:30	124.07
11/2/2011 17:00	123.91
11/2/2011 17:30	123.44
11/2/2011 18:00	123.0616
11/2/2011 18:30	123.06
11/2/2011 19:00	123.13
11/2/2011 19:30	123.745
11/2/2011 20:00	123.96
11/2/2011 20:30	123.99
11/2/2011 21:00	123.99
11/3/2011 14:30	124.3
11/3/2011 15:00	124.38
11/3/2011 15:30	124.67
11/3/2011 16:00	125.19
11/3/2011 16:30	124.9
11/3/2011 17:00	125.27
11/3/2011 17:30	125.5
11/3/2011 18:00	125.58
11/3/2011 18:30	125.91
11/3/2011 19:00	125.8
11/3/2011 19:30	125.83
11/3/2011 20:00	126.215
11/3/2011 20:30	126.25
11/3/2011 21:00	126.25
11/4/2011 14:30	124.901
11/4/2011 15:00	124.43
11/4/2011 15:30	124.4654
11/4/2011 16:00	124.46
11/4/2011 16:30	124.68
11/4/2011 17:00	124.86
11/4/2011 17:30	124.73
11/4/2011 18:00	125.22
11/4/2011 18:30	125.48
11/4/2011 19:00	125.5601
11/4/2011 19:30	125.4091
11/4/2011 20:00	125.15
11/4/2011 20:30	125.43
11/4/2011 21:00	125.481
11/7/2011 15:30	125.91
11/7/2011 16:00	125.29
11/7/2011 16:30	124.79
11/7/2011 17:00	124.77
11/7/2011 17:30	124.7
11/7/2011 18:00	124.37
11/7/2011 18:30	124.56
11/7/2011 19:00	124.86
11/7/2011 19:30	125.3
11/7/2011 20:00	125.59
11/7/2011 20:30	125.95
11/7/2011 21:00	125.73
11/7/2011 21:30	126.27
11/7/2011 22:00	126.26
11/8/2011 15:30	127.33
11/8/2011 16:00	126.37
11/8/2011 16:30	126.46
11/8/2011 17:00	126
11/8/2011 17:30	126.06
11/8/2011 18:00	126.2662
11/8/2011 18:30	126.23
11/8/2011 19:00	126.4499
11/8/2011 19:30	127.12
11/8/2011 20:00	127.48
11/8/2011 20:30	127.49
11/8/2011 21:00	127.69
11/8/2011 21:30	127.88
11/8/2011 22:00	127.88
11/9/2011 15:30	124.51
11/9/2011 16:00	124.42
11/9/2011 16:30	124.92
11/9/2011 17:00	125.18
11/9/2011 17:30	125.23
11/9/2011 18:00	124.81
11/9/2011 18:30	125.07
11/9/2011 19:00	124.61
11/9/2011 19:30	123.8869
11/9/2011 20:00	123.24
11/9/2011 20:30	123.3329
11/9/2011 21:00	123.6
11/9/2011 21:30	123.19
11/9/2011 22:00	123.161

The rownames are datas plus hour, the data column is the time series' value.

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