private static DateTime GetLastDayOfThisMonth() 
{
	DateTime now = DateTime.Now; 
	int nbDays = DateTime.DaysInMonth(now.Year, now.Month);

	return new DateTime(now.Year, now.Month, nbDays, 23, 59, 59, 999); 
}

Public Shared Function GetLastDayOfThisMonth() As DateTime 
	Dim now As DateTime = DateTime.Now
	Dim nbDays As Integer = DateTime.DaysInMonth(now.Year, now.Month) 

	Return New DateTime(now.Year, now.Month, nbDays, 23, 59, 59, 999)
End Function

<?php
function GetLastDayOfThisMonth () {
 list ($month, $year) = explode ('-', date ('m-Y'));
 $year = ((int)$month === 12)?$year+1:$year;
 $month = ((int)$month + 1 === 13)?1:$month+1;
 $lastDay = mktime (0, 0, 0, $month, 0,  $year);
 return strftime ('%d', $lastDay);
}
?>

<cffunction name="GetLastDayOfThisMonth" hint="Retourne le dernier jour du mois au format dd/mm/yyyy">
 <cfreturn DateFormat(CreateDate(Year(Now()), Month(Now()), DaysInMonth(Now())),"dd/mm/yyyy")>
</cffunction>

function GetLastDayOfThisMonth () {
 var myDate = new Date ();
 var myMonth = myDate.setMonth (myDate.getMonth() + 1);
 var theDay = myDate.setDate (0);
 var lastDay = myDate.getDate ();
 return lastDay;
}

Private Function GetLastDayOfThisMonth() As Date
     GetLastDayOfThisMonth = DateSerial(Year(Now), Month(Now) + 1, 0)
 End Function

Calendar cal = Calendar.getInstance();
int lastDayOfMonth = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
System.out.println(lastDayOfMonth);

DateDernierJour est une date
// On donne 31, si le mois en cours n'en a que 28, 29 ou 30 l'ajustement sera fait
DateDernierJour ..Jour = 31
Info("Dernier jour du mois : "+DateDernierJour ..Jour)

CREATE FUNCTION [dbo].[Last_Day] (@date AS datetime) 
RETURNS datetime
AS
BEGIN
    -- 1er jour du mois en cours
    set @date = DATEADD(DAY, 1-DAY(@date), @date)
    set @date = DATEADD(MONTH, 1, @date)
    return DATEADD(DAY, -1, @date)
END