Published TV schedules are (for various reasons) not always accurate. This means that a programme may start a little earlier or later, or (quite commonly) end later than the schedule. Therefore, most people "pad" their recordings with an extra couple of minutes before the start time and slightly more than that after the end time. This can be done by adjusting the timer manually after setting it in the EPG (press Record again and then adjust the start time and duration), but many replacement EPG TAPs offer this as an automatic feature (and can cope with the complexities of recording consecutive programmes and resolving clashes between channels).
There is a TAP, Extend, which will extend recordings based on the programme transitions signalled by the broadcaster (known as "Accurate Recording" signals in the UK). This is useful for programs which overrun significantly. It requires a particular firmware patch (already included in some customised firmware, but not in factory firmware), and depends on the broadcaster transmitting the signals correctly.