The timestamps used in OTP tokens are usually quite coarse (30 - 60 second resolution is common), also it's common to use window of few accepted timestamps arou

The timestamps used in OTP tokens are usually quite coarse (30 - 60 second resolution is common), also it's common to use window of few accepted timestamps around the valid one. (IIRC SecurID has this window in default configuration surprisingly large, on the order of tens of minutes) RTC with common 32768Hz xtal is probably accuratte enough for this application given reasonably stable temperature (ie. key stored in room temperature or near human body) with few minute acceptance window. The problem with battery life for such keyfob is that it also has relatively high powered RF transmitter and combining low permanent current consumption with high current bursts is not something that cheap primary lithium cells are optimized for (you have to pick one or the other) Edit: also given the fact that there is only one receiver and that it usually also has second bidirectional channel (immobilizer/keyless start) you can compensate for some limited clock drift in software or even periodically resynchronize the clock. Manual for Fabia III even describes procedure to "resynchronize" the keyless fob by using it as if it were ignition key, which probably does exactly this. But who knows, almost all post 2000 VWs have similar procedure for central locking remote which probably simply overwrites the counter in car with whatever next value from the keyfob as there cannot be any bidirectional communication (the old procedure involves lock in driver's door, no NFC antennas there and the imobilizer NFC chip in keyfob is completely separate from the RF part)