I would like to to take the meridian or midpoint between the stoptime and starttime and then add a new column with the midpoint. It can be rounded to the nearest second. How does one do this in R? Both stoptime and starttime are in POSIXct form. Is there an easier way than dividing difftime in half and then adding to stoptime?

> head(data)
   bikeid end.station.id start.station.id diff.time            stoptime           starttime
24  23966            359              318      1505 2015-10-13 07:45:00 2015-10-13 08:10:05
28  23966            502              311      2072 2015-10-20 14:41:11 2015-10-20 15:15:43
58  17110            337              340      3338 2015-10-15 16:00:39 2015-10-15 16:56:17
74  23822            501              527      3478 2015-10-05 15:55:13 2015-10-05 16:53:11
83  16462            426              146      3368 2015-10-01 07:52:06 2015-10-01 08:48:14
89  23121            435              223      1499 2015-10-08 11:58:08 2015-10-08 12:23:07

> dput(head(data))
structure(list(bikeid = c("23966", "23966", "17110", "23822", 
"16462", "23121"), end.station.id = c("359", "502", "337", "501", 
"426", "435"), start.station.id = c("318", "311", "340", "527", 
"146", "223"), diff.time = c(1505, 2072, 3338, 3478, 3368, 1499
), stoptime = structure(c(1444740300, 1445370071, 1444942839, 
1444078513, 1443703926, 1444323488), class = c("POSIXct", "POSIXt"
), tzone = "EST"), starttime = structure(c(1444741805, 1445372143, 
1444946177, 1444081991, 1443707294, 1444324987), class = c("POSIXct", 
"POSIXt"), tzone = "EST")), .Names = c("bikeid", "end.station.id", 
"start.station.id", "diff.time", "stoptime", "starttime"), row.names =      c(24L, 28L, 58L, 74L, 83L, 89L), class = "data.frame")