The next version of the standard (v5) has the proposed keywords formatMaximum
/formatMinimum
which would do exactly what you wanted, in an elegant way: they combine with format
to provide format-specific min/max.
Currently (with v4), your only option is to create a hack using pattern
(e.g. ^20(0[6-9]|1[0-2])
). However, this is completely horrible, and best avoided if possible.