what is one indication that time has passed on the island?

Check out the second paragraph in chapter 3,

"His sandy hair, considerably longer than it had been when they dropped in, was lighter now; and his back was a mass of dark freckles and peeling sunburn."

Later on Ralph notices that he needs a haircut.

Simon is also described with deep sunburns and long hair, ..."The coarse mop of black hair was long and swung down... He wore the remains of shorts....and, Always darkish in colour, Simon was burned by the sun to a deep tan."

These boys are British Military school boys so they would have had very short haircuts to begin with.