You can use the CONVERT function to cast a datetime datatype to a formatted string.

SELECT GETDATE() AS [Result] -- 2016-07-21 07:56:10.927

You can also use some built-in codes to convert into a specific format. Here are the options built into SQL Server:

DECLARE @convert_code INT = 100 -- See Table Below
SELECT CONVERT(VARCHAR(30), GETDATE(), @convert_code) AS [Result]

SELECT GETDATE() AS [Result]                                -- 2016-07-21 07:56:10.927
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),100) AS [Result] -- Jul 21 2016  7:56AM
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),101) AS [Result] -- 07/21/2016
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),102) AS [Result] -- 2016.07.21
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),103) AS [Result] -- 21/07/2016
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),104) AS [Result] -- 21.07.2016
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),105) AS [Result] -- 21-07-2016
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),106) AS [Result] -- 21 Jul 2016
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),107) AS [Result] -- Jul 21, 2016
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),108) AS [Result] -- 07:57:05
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),109) AS [Result] -- Jul 21 2016  7:57:45:707AM
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),110) AS [Result] -- 07-21-2016
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),111) AS [Result] -- 2016/07/21
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),112) AS [Result] -- 20160721
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),113) AS [Result] -- 21 Jul 2016 07:57:59:553
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),114) AS [Result] -- 07:57:59:553
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),120) AS [Result] -- 2016-07-21 07:57:59
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),121) AS [Result] -- 2016-07-21 07:57:59.553
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),126) AS [Result] -- 2016-07-21T07:58:34.340
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),127) AS [Result] -- 2016-07-21T07:58:34.340
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),130) AS [Result] -- 16 ???? 1437  7:58:34:340AM
UNION SELECT CONVERT(VARCHAR(30),GETDATE(),131) AS [Result] -- 16/10/1437  7:58:34:340AM

The built-in functions GETDATE and GETUTCDATE each return the current date and time without a time zone offset.

The return value of both functions is based on the operating system of the computer on which the instance of SQL Server is running.

The return value of GETDATE represents the current time in the same timezone as operating system. The return value of GETUTCDATE represents the current UTC time.

Either function can be included in the SELECT clause of a query or as part of boolean expression in the WHERE clause.

Examples:

-- example query that selects the current time in both the server time zone and UTC
SELECT GETDATE() as SystemDateTime, GETUTCDATE() as UTCDateTime

-- example query records with EventDate in the past.
SELECT * FROM MyEvents WHERE EventDate < GETDATE() 

There are a few other built-in functions that return different variations of the current date-time:

SELECT 
    GETDATE(),          --2016-07-21 14:27:37.447
    GETUTCDATE(),       --2016-07-21 18:27:37.447
    CURRENT_TIMESTAMP,  --2016-07-21 14:27:37.447
    SYSDATETIME(),      --2016-07-21 14:27:37.4485768
    SYSDATETIMEOFFSET(),--2016-07-21 14:27:37.4485768 -04:00
    SYSUTCDATETIME()    --2016-07-21 18:27:37.4485768

General syntax:

DATEADD (datepart , number , datetime_expr)  

To add a time measure, the number must be positive. To subtract a time measure, the number must be negative.

Examples

DECLARE @now DATETIME2 = GETDATE();
SELECT @now;                        --2016-07-21 14:39:46.4170000
SELECT DATEADD(YEAR, 1, @now)       --2017-07-21 14:39:46.4170000
SELECT DATEADD(QUARTER, 1, @now)    --2016-10-21 14:39:46.4170000
SELECT DATEADD(WEEK, 1, @now)       --2016-07-28 14:39:46.4170000
SELECT DATEADD(DAY, 1, @now)        --2016-07-22 14:39:46.4170000
SELECT DATEADD(HOUR, 1, @now)       --2016-07-21 15:39:46.4170000
SELECT DATEADD(MINUTE, 1, @now)     --2016-07-21 14:40:46.4170000
SELECT DATEADD(SECOND, 1, @now)     --2016-07-21 14:39:47.4170000
SELECT DATEADD(MILLISECOND, 1, @now)--2016-07-21 14:39:46.4180000

NOTE: DATEADD also accepts abbreviations in the datepart parameter. Use of these abbreviations is generally discouraged as they can be confusing (m vs mi, ww vs w, etc.).

These are the datepart values available to date & time functions:

NOTE: Use of abbreviations is generally discouraged as they can be confusing (m vs mi, ww vs w, etc.). The long version of the datepart representation promotes clarity and readability, and should be used whenever possible (month, minute, week, weekday, etc.).

General syntax:

DATEDIFF (datepart, datetime_expr1, datetime_expr2)

It will return a positive number if datetime_expr is in the past relative to datetime_expr2, and a negative number otherwise.

Examples

DECLARE @now DATETIME2 = GETDATE();
DECLARE @oneYearAgo DATETIME2 = DATEADD(YEAR, -1, @now);
SELECT @now                                    --2016-07-21 14:49:50.9800000
SELECT @oneYearAgo                             --2015-07-21 14:49:50.9800000
SELECT DATEDIFF(YEAR, @oneYearAgo, @now)       --1
SELECT DATEDIFF(QUARTER, @oneYearAgo, @now)    --4
SELECT DATEDIFF(WEEK, @oneYearAgo, @now)       --52
SELECT DATEDIFF(DAY, @oneYearAgo, @now)        --366
SELECT DATEDIFF(HOUR, @oneYearAgo, @now)       --8784
SELECT DATEDIFF(MINUTE, @oneYearAgo, @now)     --527040
SELECT DATEDIFF(SECOND, @oneYearAgo, @now)     --31622400

NOTE: DATEDIFF also accepts abbreviations in the datepart parameter. Use of these abbreviations is generally discouraged as they can be confusing (m vs mi, ww vs w, etc.).

DATEDIFF can also be used to determine the offset between UTC and the local time of the SQL Server. The following statement can be used to calculate the offset between UTC and local time (including timezone).

select  DATEDIFF(hh, getutcdate(), getdate()) as 'CentralTimeOffset'

DATEPART returns the specified datepart of the specified datetime expression as a numeric value.

DATENAME returns a character string that represents the specified datepart of the specified date. In practice DATENAME is mostly useful for getting the name of the month or the day of the week.

There are also some shorthand functions to get the year, month or day of a datetime expression, which behave like DATEPART with their respective datepart units.

Syntax:

DATEPART ( datepart , datetime_expr )
DATENAME ( datepart , datetime_expr )
DAY ( datetime_expr )
MONTH ( datetime_expr )
YEAR ( datetime_expr )

Examples:

DECLARE @now DATETIME2 = GETDATE();
SELECT @now                       --2016-07-21 15:05:33.8370000
SELECT DATEPART(YEAR, @now)       --2016
SELECT DATEPART(QUARTER, @now)    --3
SELECT DATEPART(WEEK, @now)       --30
SELECT DATEPART(HOUR, @now)       --15
SELECT DATEPART(MINUTE, @now)     --5
SELECT DATEPART(SECOND, @now)     --33
-- Differences between DATEPART and DATENAME:
SELECT DATEPART(MONTH, @now)      --7
SELECT DATENAME(MONTH, @now)      --July
SELECT DATEPART(WEEKDAY, @now)    --5
SELECT DATENAME(WEEKDAY, @now)    --Thursday
--shorthand functions
SELECT DAY(@now)    --21
SELECT MONTH(@now)  --7
SELECT YEAR(@now)   --2016

NOTE: DATEPART and DATENAME also accept abbreviations in the datepart parameter. Use of these abbreviations is generally discouraged as they can be confusing (m vs mi, ww vs w, etc.).

Using the DATEADD and DATEDIFF functions, it's possible to return the last date of a month.

SELECT DATEADD(d, -1, DATEADD(m, DATEDIFF(m, 0, '2016-09-23') + 1, 0))
-- 2016-09-30 00:00:00.000

SELECT EOMONTH('2016-07-21')        --2016-07-31
SELECT EOMONTH('2016-07-21', 4)     --2016-11-30
SELECT EOMONTH('2016-07-21', -5)    --2016-02-29

There are many ways to return a Date from a DateTime object

1. SELECT CONVERT(Date, GETDATE())
2. SELECT DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE())) returns 2016-07-21 00:00:00.000
3. SELECT CAST(GETDATE() AS DATE)
4. SELECT CONVERT(CHAR(10),GETDATE(),111)
5. SELECT FORMAT(GETDATE(), 'yyyy-MM-dd')

Note that options 4 and 5 returns a string, not a date.

This function will take 2 datetime parameters, the DOB, and a date to check the age at

  CREATE FUNCTION [dbo].[Calc_Age]
    (
    @DOB datetime , @calcDate datetime 
    )
    RETURNS int
    AS
   BEGIN
declare @age int

IF (@calcDate < @DOB  )
RETURN -1

-- If a DOB is supplied after the comparison date, then return -1
SELECT @age = YEAR(@calcDate) - YEAR(@DOB) +
  CASE WHEN DATEADD(year,YEAR(@calcDate) - YEAR(@DOB)
  ,@DOB) > @calcDate THEN -1 ELSE 0 END
    

RETURN @age
    

END

eg to check the age today of someone born on 1/1/2000

SELECT  dbo.Calc_Age('2000-01-01',Getdate())