Missing values are not permitted in coordinate variables. A time coordinate variable will be identifiable by its units, alone. The units attribute will be of character type with the string formatted as per the recommendations in the Unidata udunits package version 1.7.1. The acceptable units for time are listed in the file udunits.dat. Plural forms are also acceptable.
Year is an ambiguous unit as years are of varying length.
Udunits defines a year as exactly 365 days. A time coordinate variable is identifiable from its units string, alone. The udunits routines utScan and utIsTime can be used to make this determination. The units attribute will be of character type. Plural forms are also acceptable.