Is this what you are looking for? A formula to calculate the last day of the month constantly?
You have to put these in seprate cells, but you can hide them and only show the EOM formula. today() is in cell A1 in this example.
<TABLE style="WIDTH: 209pt; BORDER-COLLAPSE: collapse" border=0 cellSpacing=0 cellPadding=0 width=278><COLGROUP><COL style="WIDTH: 96pt" width=128><COL style="WIDTH: 113pt; mso-width-source: userset; mso-width-alt: 2742" width=150><TBODY><TR style="HEIGHT: 15pt" height=20><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 96pt; HEIGHT: 15pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" height=20 width=128>=TODAY()</TD><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 113pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl63 width=150></TD></TR><TR style="HEIGHT: 15pt" height=20><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 15pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" height=20>=DAY(A1)</TD><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8"></TD></TR><TR style="HEIGHT: 15pt" height=20><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 15pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" height=20>=MONTH(A1)</TD><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8"></TD></TR><TR style="HEIGHT: 15pt" height=20><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 15pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" height=20>=YEAR(A1)</TD><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl63>=EOMONTH(DATE($A$4,$A$3,$A$2),0)</TD></TR></TBODY></TABLE>