What you do depends upon past experience and the insulin you are using. Lantus (and maybe Levemir) is not that amendable for early/late shot. By history means is cat susceptible to ketones? If yes then missing a dose is not that good. A typical curve (BG's every two hours) is also good information to have. A good solution is to shoot late and then shoot 11 hours 45 minutes early each time until regular shot times are reestablished.
Also, it would have been better to post on the diabetic health forum since this is a diabetic cat issue. You posed on the general health (non diabetic) forum.