Create visit values based on time

#1
I am trying to create a visit number based on time. The data I have currently looks like this:

Date Time Cow
11/1/15 2:00 77
11/1/15 2:01 77
11/1/15 2:02 77
11/1/15 2:03 77
11/1/15 2:04 77
11/1/15 2:05 77
11/1/15 2:06 77
11/1/15 4:00 77
11/1/15 4:01 77
11/1/15 4:02 77
11/1/15 4:03 77
11/1/15 4:04 77
11/1/15 4:05 77
11/1/15 9:11 77
11/1/15 9:12 77
11/1/15 9:13 77
11/1/15 9:14 77
11/1/15 9:15 77

This is overly simplified, as the times go out to hh:mm:ss.0 and there is about 20 minutes per visit (these are cows entering a milking unit and being milked over a period of time). I would like to separate these visits out with a number like below:

Date Time Cow Visit
11/1/15 2:00 77 1
11/1/15 2:01 77 1
11/1/15 2:02 77 1
11/1/15 2:03 77 1
11/1/15 2:04 77 1
11/1/15 2:05 77 1
11/1/15 2:06 77 1
11/1/15 4:00 77 2
11/1/15 4:01 77 2
11/1/15 4:02 77 2
11/1/15 4:03 77 2
11/1/15 4:04 77 2
11/1/15 4:05 77 2
11/1/15 9:11 77 3
11/1/15 9:12 77 3
11/1/15 9:13 77 3
11/1/15 9:14 77 3
11/1/15 9:15 77 3

Doing this will allow me to summarize different measurements by visit instead of by day. Any ideas would be greatly appreciated. Thank you!! :yup: