Er... Not quite sure I understand.
Your PC checks the time from a "web site" usually once a week. Click the time in the system tray and have a look at the settings. In between these checks it runs from a small battery backup and is accurate to a few seconds a week.
Digital clocks ! just keep time from a quartz oscillator, some are more accurate than others but still within a few seconds a week.
Another type of clock has a built in radio receiver that checks (once an hour usually) for the exact time from a radio station that broadcasts data from an atomic clock... not a normal radio station. Again it has the quartz reference so in between checks runs as a normal clock.