Including Function

1. convert 2-digit year to 4-digit year
2. convert year, day of year to GPS week, day of week
3. convert GPS week, day of week to year, day of year
4. convert year, day of year to month, day
5. convert GPS week, seconds of week to modified Julian date (MJD)
6. convert year, month, day or year, day of year to modified Julian date (MJD)
7. convert modified Julian date (MJD) to year, day of year
8. to get the week day