Actually, your best bet might be a micro-controller, if you're willing to do some programming; you would want to use a servo rather than a solenoid (easier to have a dual-state lock/unlock). You could even easily incorporate an LCD showing all sorts of extra info, allow changing of open periods (you could program it to open multiple times in a day, for instance, if needed). Your choice of micro-controller could be nearly anything; an Arduino, Basic Stamp, AVR (of which the Arduino is a variant), or PIC would work fine. If you are new to micro-controllers, the Arduino might be the easiest to get your head around, as it was designed for use by non-engineers and artists.