These dates are the dates you must enter Thailand otherwise the visa will lapse. Once you arrive your passport will be stamped with the date of entry, the last date the visa is still valid (+60 days… + 90 days …) and the visa type. I assume it is a non-O as you would’ve received 90 days exempt in any case.
My TM30 issued by the landlord/ agent states the duration of the stay as per annual lease. The TM30 has been scrutinised, compared with the actual lease and stamped by local immigration office. I assume there is no need to issue another one whenever i return from local holidays staying in hotels or returning from abroad.
You can be denied entry solely due to having no onward flight out of Thailand at the end of the holiday/ stay/ visa period at which point the airline which brought you into the country can become liable for removal. If you are denied entry for any other reasons the airline cannot be responsible and you have to fund the return yourself or being thrown into immigration detention until you find funds to do so.