Below is the one by one description of all the four date command with their syntax.
GETDATE():
GetDate() Command is used to get the current date and time of the system from standard. The syntax of GetDate() is:
SELECT GETDATE();
[Returns current date and time]
DATEDIFF():
DateDiff() Command is used to fetch the difference between the two different dates. Here is the syntax:
DATEDIFF ( DatePart, StartDate, EndDate);
DATEADD():
DateAdd() is used to add or delete the specified time interval you want to add. The abbreviations used for datepart are:
Year = yy, yyyy
Month = mm
Day = dd, d
Week = wk,ww
Hour = hh
Minute = min
Second = ss, s
Milisecond = ms
DATEPART():
DatePart() is helpful for getting a single part of a date/time. The Syntax is:
DATEPART (DatePart, Date);