POSIXct values are simply the number of seconds since midnight GMT 1970-01-01. (So you need to pay attention to your offset from UTC.) You can use the date part and add the number of days times 24*3600 (as.Date(dtval) to your time value * 24*3600. Gabor pointed to the article in R News (which he wrote, thank you, Gabor.)
You didn't give an example of the string. If you are getting your date as a string, then as.Date(strDate) will convert a variable "strDate" to Date class when it is in either "YYYY-MM-DD" or "YYYY/MM/DD" format. Otherwise the formatting codes are on the ?strptime page.
Once you have a POSIXct-classed variable you can just add the number of seconds. This example add 30 minutes to midnight today Feb 1, 2011 (in my time zone which is UTC-5):
> as.POSIXct(as.Date("2011-02-01")) +30*60
[1] "2011-01-31 19:30:00 EST"
And this is your time value added to midnight my time:
> as.POSIXct(as.Date("2011-02-01 00:00", tzone="UTC"))+3600*5 + 3600*24*timeval
[1] "2011-02-01 03:29:59 EST"
