Year zero
Year zero does not exist in the anno Domini system usually used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year is followed by . However, there is a year zero in astronomical year numbering (where it coincides with the Julian year ) and in ISO 8601:2004 (where it coincides with the Gregorian year ) as well as in all Buddhist and Hindu calendars.
WWV (radio station)
Like the WWVB time code, only the tens and units digits of the year are transmitted; 
unlike the WWVB time code, there is no direct indication for leap year. 
Thus, receivers assuming that year 00 is a leap year (correct for year 2000) will be incorrect in the year 2100.
On the other hand, receivers that assume year 00 is not a leap year will be correct for 2001 through 2399.