In western countries new date begins at 12 a.m. Maximum number of days in one month is 31. The month of February has 28 or 29 days. January, March, May, July, August, October and December are the months of 31 days and April, June, September and November are the months of 30 days. This calender is used by the whole world.
In ancient Indian calendars one month was of 27 days, in which days were considered by the names of Nakshatras. Then each month was divided into weeks and days were considered by the names of planets, which continues even today.
In India Moon month is the basis of Tithis, in which one month is divided into two halves. One is bright half and the second is dark half. Each half has 15 days. One Tithi forms when Sun and Moon are placed at a difference of 13Âº-20. Tithis in the bright half begin from Pratipada to full Moon day and in the dark half, Tithis begin from Pratipada to no Moon day.
One tithi begins with Sun rise. Tithi will be effective if it is present at the time of Sun rise, but if a Tithi begins after Sun rise and finishes before next Sunrise, then that Tithi will not be considered as one day. Sometimes one Tithi extends from one Sun rise to next Sun rise, that Tithi is known as increased Tithi. People should not perform auspicious events in these types of tithis.
In India, all the Sanskaras are related to spirituality. Therefore each Tithi has a deity. In Hindu calender Tithis can increase or decrease, but in western calendar dates cannot decrease or increase.