More fun with sed

So I have this strange date and time string, which I would like to convert to a “useable” date, i.e., something that a spreadsheet programme or R can work with. It looks like this (MON has 3 chars):


The string is the second field in a csv file, preceded and followed by a comma.

My strategy was to terminate the string before the first colon and delete everything thereafter to be left with the following string (with one occurrence in each of the about 6000 lines of the file):

sed does this in a single line (looks kinda ugly, but does the trick):
sed 's/:[0-9][0-9]:[0-9][0-9]:[0-9][0-9]//g' myfile.csv >myfile2.csv