As Fruit Bat /\0/\ has stated, STBs and the like set their clocks from transmitted signals. It is possibble, however, that once set the clock is updated from an internal oscillator which will depart from true time over a period. If that applies and if you keep your box permanently on standby or supplied with mains power, it might be worth disconnecting from the mains for a while. On reconnecting the clock should then be updated from transmitted time. I assume that a factory reset would do the same.