Time is measured in small frames of day and night as well as in larger frames of years marking the change of seasons. The yearly measurement of Time is called a calendar.
There are three main types of calendar. The solar calendar based on the number of days the earth takes to move around the sun. It is roughly 365 days and 6 hours. To accomodate for these extra 6 hours, a day is added every 4 years to account for the extra hours. The year when the day is added is called a leap year. Here months are divided bit arbitrarily, only roughly corresponding to the movement of the sun, moon and earth through the constellation of stars (zodiac).
There is also the lunar calendar which takes a month as the cycle of the moon around the earth which is 28 days. So here the year is shorter than the solar calendar. It is followed by Islamic world.
A good combination of the two, lunar-solar calendar, is the Hindu, Chinese and Jewish calendar. This is the most accurate calendar taking into consideration the year, month, seasons.
Affectionately,
Alok Da
