If you have proof that it was available to rent, that's your placed-in-service date when expenses start.  A few months is not unusual, if it were years I'd have more trouble claiming expenses (but even then, depending on how persuasive the "proof" is . . . here's the contract signed with the listing agent, here's a printout of the listing and the dates it was posted, here's a calendar showing dates and times the property was shown to prospective tenants . . .)