[yyyy:+xx] : Year Offset Suffix
Tokens > Year Tokens >
Use the Year Offset suffix to display a year that is offset by +xx years, or -xx years from the value of the token that precedes it. This suffix can be added to either the [yy], [yyyy] or [year] tokens.
See also : Year of Now, Script Year, Year of parent Minicalendar, Minical Year Offset Modifier.