In addition to @DefinitelyNot31337's definitely genius solution, in case you're wondering how to get the last day of the current month, you may to use this syntax:
DATEADD(
month,
1,
DATEADD(
DAY,
-DAY('[@cbTimestamp*]'),
'[@cbTimestamp*]'
)
)
Hope this helps,
TroubleShooter