How Do I Parse a Date Time String That Includes Fractional Time?

I have a date time string:

20:48:01.469 UTC MAR 31 2016

I would like to convert this string representation of time to a struct tm using strptime, but my format string isn't working.

Is there a format specifier for fractional seconds? Perhaps %S, %s, or something else?

Code snippet is below:

tm tmbuf;
const char *str = "20:48:01.469 UTC MAR 31 2016"
const char *fmt = "%H:%M:%s %Z %b %d %Y";
strptime(str,fmt,&tmbuf);