The boys keep track of time by using the sun. Morning, noon, and evening are fairly straightforward. The boys have no need for minute by minute time keeping any longer. Their priorities are survival and rescue. By day the boys do most of their hunting and working, and night is shelter and sleep. The sun also serves a second time keeping function beyond guiding daily routine. It gives the boys an accurate way to count the number of days that they have been on the island.
Piggy does suggest making a sundial, and attempts to explain how it could be made from sticks. Ralph and a few of the other boys don't see the point and more or less tell Piggy to forget about it.
“I’ve been thinking,” he said, “about a clock. We could make a sundial. We could put a stick in the sand, and then—” . . .
“We got a lot of sticks. We could have a sundial each. Then we should know what the time was.”
“A fat lot of good that would be.”
“You said you wanted things done. So as we could be rescued.”
“Oh, shut up.”