Lunar and Solar years
It is generally believed that a Lunar year is smaller than a Solar year by about 10 days.
It is given in the Qoran that some people were in a cave then they went to sleep and Allah caused them to remain asleep for 300 years and for 9 more years. When they awoke they thought that they had been sleeping for only a day or so. See full surah 18 (Alkahaf)
ولبثو ا في كهفهم ثلثه ماأته سنين و از دادو تسعا
Translation by Sahih International
And they remained in their cave for three hundred years and exceeded by nine.
Translation in Urdu
اور اصحاب کہف اپنے غار میں نو اوپر تین سو سال رہے
Now my friend Prof. Muhammad Umar Chand told me that Allah gave these figures in both Solar and Lunar Calendars. He said that 300 years in Solar calendar are equal to 309 Lunar years. I have tried to confirm this observation below.
One lunar month = 29.531 days (taken from internet)
One lunar year = 29.531 x 12 = 354.372 days
one solar year = 365.25 days
300 solar years x 365.25 days = 109575 days
Now 109575 days / 354.372 = 309.2 lunar years!
Allah knows best why He said 300 years and 9 more years and NOT directly three hundred and nine years. But the above seems to be very credible reason.