I have a data set with times as strings representing minutes and seconds in the format mm:ss.s. It currently looks like:
You should be able to use strptime with a custom format string like so:
> strptime("00:14.3", format="%M:%OS")  "2016-08-24 00:00:14 PDT"
Note that this will create a full datetime object using your current date and timezone. To see that the fractional second has been preserved, make sure your options(digits.secs) is set like this:
> options(digits.secs=2) > strptime("00:14.3", format="%M:%OS")  "2016-08-24 00:00:14.30 PDT"
You can adjust the date and timezone manually or with the strptime options. Depending on how you're using this, you can always format that back out into a string (reproduced here to prove no information was lost in the conversion):
> strftime(t, format="%M:%OS")  "00:14.30"