NTP automatically corrects for clock drift as your machine is running. Clock drift is inevitable, and on some machines it can be very drastic. By synchronizing your clock with an NTP server, you will guarantee accurate time displays on the frontend OSD and make sure that recordings always start on time on the backend. With NTP running, you have the best chance of never missing recordings- as long as your MythTV configuration is correct and the broadcaster is on time.
To begin, perform a one time major sync using ntpdate, which will correct the clock even if its a drastic problem like being off by several hours.
# sudo ntpdate ntp.ubuntu.com
After setting the clock correctly the first time, install the ntp-simple package. This includes a ntp daemon that will periodically connect to a NTP server to make sure that your clock is in sync.
# sudo apt-get install ntp-simple
If after some time, you find that the accuracy of the clock is not acceptable, you can add more servers for increased accuracy. See NTPTimeSynchronisation for more information.