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.