Add functionalities related to dates.

[nstarlike]

We sometimes need to get dates like today, yesterday and the dates before or after 1 month, 1 year, etc while we develop some applications. To get those dates is not that simple becuase the counts of the days of a month and a year are different. For example, the days of January is 31 and the days of April is 30. And when we calculate the days of February we need to consider leap year. If the year is leap year then the days will be 29 if not it will be 28. Lastly, we need to consider whether we want to include the basis date.
The following is the list for functionalities needed.

• To get today.
• To get yesterday.
• To get tomorrow.
• To get the date x month(s) before and later.
• To get the date x year(s) before and later.
• To calculate the days between dates.
• To calculate the months between dates.
• To calculate the years between dates.

Go for it!