It also helps to have a Julian date calendar, if you have a date code like '1368', which is the 136th day of '68. Try:

http://www.imap.mesacounty.us/GPS_Survey/RINEX%20TIME/julian-calendar.pdf

There are two calendars, one for leap years, and one for others. Of course, leap years are the ones evenly divisible by 4, such as 1968 and 1972.