A locked portal unlocking control system (21) including an encryption device (22) responsive to input of a structure identifier to encrypt a time representation with an encrypting cryptographic key (37) for the structure to produce a cryptogram. At least one structure (32, 32a, 32b, . . . 32n) is remote of the encryption device (22) having a lock mechanism (26) controlling opening of a portal to the structure. Each structure (32, 32a, 32b, . . . 32n) further includes a decryption device (27, 27a, 27b, . . . . 27n) having an unlocking assembly (54) coupled to the lock mechanism (26) with the decryption device (27 27n) being responsive to input of the cryptogram (24) to unlock the lock mechanism (26) if a decrypted time representation produced by decrypting the cryptogram meets a time-based criteria in the decryption device (27 27n). A method is also disclosed including the steps of creating a cryptogram, transporting (82) the cryptogram (24) to a remote structure (32 32n), inputting the cryptogram (24) into a decryption device (27) at the structure, decrypting the cryptogram to produce a decrypted time representation (83), comparing (84) the decrypted time representation with a time-based criteria (61) and unlocking the lock (54) if the decrypted time representation (83) meets the time-based criteria (61).