split or subset data into 30 minute intervals
问题 I have a data frame of the following form: Temp Depth Light x time date time.at.depth 104 18.59 -2.7 27 21:38 2012-06-20 4 109 18.59 -2.7 27 22:02 2012-06-20 5 110 18.75 -4.0 27 22:07 2012-06-20 5 113 18.91 -2.7 27 22:21 2012-06-20 4 114 18.91 -4.0 27 22:26 2012-06-20 5 115 18.91 -2.7 27 22:31 2012-06-20 5 117 18.91 -2.7 27 22:40 2012-06-20 4 118 18.75 -4.0 27 22:45 2012-06-20 5 119 18.75 -2.7 27 22:50 2012-06-20 5 121 18.59 -4.0 27 22:59 2012-06-20 4 122 18.75 -2.7 27 23:04 2012-06-20 5 123