Abstract: A locking device comprising: a code generation means for generating a plurality of access codes in a first series and a second series, each access code being valid for a predetermined period of time, a code input means for receiving an input code, and a code comparison means, wherein the code comparison means is configured to unlock the lock in response to input of a code that corresponds to a currently valid access code, wherein the period of validity of each access code in first series partially overlaps the period of validity two adjacent access codes in the second series.