Many motherboards contain pads for trim caps and the computer vendors don't use them. Many TSR's are available from the internet which do this. Here is a 500byte TSR which updates the system time from the RTC every 30 minutes. If the PC or terminal is networked, updating it periodically over the network is very common. Generally, the server would obtain the time from an extremely reliable source and then update all of the terminals.
Many utilities exist for obtaining extremely accurate times via modems etc.
An interesting finding was that all of our company's 486 class PCs had system time skew relative to the RTC from between 5-35 seconds in a day. However, our Gateway Pentiums exhibit absolutely no time skew between the RTC and the system time throughout the day. It would be interesting to find out. Rather than calling int1A functions, the time and date information could be read directly out of the RTC in a similar manner. But this may be difficult.