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.
On my Panasonic DVD recorder, when you retune (eg when the analogue signal is switched off) for some reason the auto detect of time switches off. In the menu - one of the sub folders of the set up menu - there is the option to turn on the auto detect of time. Not sure if this applies to your machine.I only found this out when it recorded the wrong programme after a retune (we have had 3 of these in my area recently)
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