How to determine the number of days in a month in SQL Server?

You can use the following with the first day of the specified month:

datediff(day, @date, dateadd(month, 1, @date))

To make it work for every date:

datediff(day, dateadd(day, 1-day(@date), @date),
              dateadd(month, 1, dateadd(day, 1-day(@date), @date)))

--Last Day of Previous Month
SELECT DATEPART(day, DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE()),0)))

--Last Day of Current Month
SELECT DATEPART(day, DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())+1,0)))

--Last Day of Next Month
SELECT DATEPART(day, DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())+2,0)))

Personally though, I would make a UDF for it if there is not a built in function...

   --- sql server below 2012---
    select day( dateadd(day,-1,dateadd(month, 1, convert(date,'2019-03-01'))))
    -- this for sql server 2012--
    select day(EOMONTH(getdate()))

You do need to add a function, but it's a simple one. I use this:

CREATE FUNCTION [dbo].[ufn_GetDaysInMonth] ( @pDate    DATETIME )

RETURNS INT
AS
BEGIN

    SET @pDate = CONVERT(VARCHAR(10), @pDate, 101)
    SET @pDate = @pDate - DAY(@pDate) + 1

    RETURN DATEDIFF(DD, @pDate, DATEADD(MM, 1, @pDate))
END

GO

SELECT Datediff(day,
(Convert(DateTime,Convert(varchar(2),Month(getdate()))+'/01/'+Convert(varchar(4),Year(getdate())))),
(Convert(DateTime,Convert(varchar(2),Month(getdate())+1)+'/01/'+Convert(varchar(4),Year(getdate()))))) as [No.of Days in a Month]

select add_months(trunc(sysdate,'MM'),1) -  trunc(sysdate,'MM') from dual;

I upvoted Mehrdad, but this works as well. :)

CREATE function dbo.IsLeapYear
(
    @TestYear int
)
RETURNS bit
AS
BEGIN
    declare @Result bit
    set @Result = 
    cast(
        case when ((@TestYear % 4 = 0) and (@testYear % 100 != 0)) or (@TestYear % 400 = 0)
        then 1
        else 0
        end
    as bit )
    return @Result
END
GO

CREATE FUNCTION dbo.GetDaysInMonth
(
    @TestDT datetime
)
RETURNS INT
AS
BEGIN

    DECLARE @Result int 
    DECLARE @MonthNo int

    Set @MonthNo = datepart(m,@TestDT)

    Set @Result = 
    case @MonthNo
        when  1 then 31
        when  2 then 
            case 
                when dbo.IsLeapYear(datepart(yyyy,@TestDT)) = 0
                then 28
                else 29
            end
        when  3 then 31
        when  4 then 30
        when  5 then 31
        when  6 then 30
        when  7 then 31
        when  8 then 31
        when  9 then 30 
        when 10 then 31
        when 11 then 30 
        when 12 then 31
    end

    RETURN @Result
END
GO

To Test

declare @testDT datetime;

set @testDT = '2404-feb-15';

select dbo.GetDaysInMonth(@testDT)

here's another one...

Select Day(DateAdd(day, -Day(DateAdd(month, 1, getdate())), 
                         DateAdd(month, 1, getdate())))

DECLARE  @m int
SET     @m = 2

SELECT 
       @m AS [Month],
       DATEDIFF(DAY,
                DATEADD(DAY, 0, DATEADD(m, +@m -1, 0)),
                DATEADD(DAY, 0, DATEADD(m,+ @m, 0))
               ) AS [Days in Month]

RETURN day(dateadd(month, 12 * @year + @month - 22800, -1)) 
select day(dateadd(month, 12 * year(date) + month(date) - 22800, -1)) 

A cleaner way of implementing this is using the datefromparts function to construct the first day of the month, and calculate the days from there.

CREATE FUNCTION [dbo].[fn_DaysInMonth]
(
    @year INT,
    @month INT
)
RETURNS INT
AS
BEGIN

IF @month < 1 OR @month > 12 RETURN NULL;
IF @year < 1753 OR @year > 9998 RETURN NULL;

DECLARE @firstDay DATE = datefromparts(@year, @month, 1);
DECLARE @lastDay DATE = dateadd(month, 1, @firstDay);

RETURN datediff(day, @firstDay, @lastDay);

END
GO

Similarily, you can calculate the days in a year:

CREATE FUNCTION [dbo].[fn_DaysInYear]
(
    @year INT
)
RETURNS INT
AS
BEGIN

IF @year < 1753 OR @year > 9998 RETURN NULL;

DECLARE @firstDay DATE = datefromparts(@year, 1, 1);
DECLARE @lastDay DATE = dateadd(year, 1, @firstDay);

RETURN datediff(day, @firstDay, @lastDay);

END
GO

DECLARE @Month INT=2,
    @Year INT=1989
DECLARE @date DateTime=null
SET @date=CAST(CAST(@Year AS nvarchar) + '-' + CAST(@Month AS nvarchar) + '-' + '1' AS DATETIME);

DECLARE @noofDays TINYINT 
DECLARE @CountForDate TINYINT
SET @noofDays = DATEPART(MONTH,@date )
SET @CountForDate = 28 + (@noofDays + floor(@noofDays/8)) % 2 + 2 %    @noofDays + 2 * floor(1/@noofDays)   
SET @noofDays= @CountForDate + CASE WHEN @CountForDate = 28 AND DATEPART(YEAR,@date)%4 =0 THEN 1 ELSE 0 END
PRINT @noofDays