How do I get the last day on the month using SQL Reporting Services

I know you've found your own answer, but I'd suggest this alternative:

=Today.AddDays(1-Today.Day).AddMonths(1).AddDays(-1)

It's a little easier to read, in my opinion, and might have slightly better performance (though most likely unnoticeable)

And, of course, if you wanted to pad out that date to 23:59:59, as is often necessary, just modify slightly:

=Today.AddDays(1-Today.Day).AddMonths(1).AddSeconds(-1)

Here is the answer I came up with

=DateSerial(Year(Now()), Month(Now()), "1").AddMonths(1).AddDays(-1)

As it took me a while to Figure this out, and JC's answer was the simplest to modify and had a good logical structure. I expanded it Slightly for others searching for answers on this topic. I have found normally you don't just want the Last Day of a month / year you also want the first day of the month / year. So here they are the full lines to calculate just that. wtih the SQl version there also.

First Day of Current month

SSRS=Today.AddDays(1-Today.Day)
SQL=SELECT DATEADD(s,0,DATEADD(mm, DATEDIFF(m,0,getdate()),0))

Last day of Current Month

SSRS=Today.AddDays(1-Today.Day).AddMonths(1).AddDays(-1)
SQL=SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())+1,0))

First Day of Current year

SSRS=Today.AddMonths(1-Today.month).AddDays(1-Today.day)
SQL=SELECT DATEADD(yy, DATEDIFF(yy,0,getdate()), 0)

Last Day of Current Year

SSRS=Today.AddDays(1-Today.Day).AddMonths(13-today.month).AddDays(-1)
SQL=SELECT DATEADD(dd,-1,DATEADD(yy,0,DATEADD(yy,DATEDIFF(yy,0,getdate())+1,0)))

I hope this helps somone.