Such notice may be served personally on the offender and owner or
by mailing a copy thereof to the last known address of the offender
and of the owner and by posting a copy thereof at the premises where
the violation occurs. Such notice shall state when, not less than
20 days after such service, the condition or thing which constitutes
the violation shall be corrected or removed. Proof of any personal
service or of such mailing and posting shall be sufficient proof of
such service, and the date stated therein shall become the first date
of such violation if continued.