I'm trying to transform a number "200412" into the date of the last day of
that month. If A1 = "200412", I do below steps. Step 1 works fine, but in
Step 2, I can't figure out a formula to return "12/31/2004" or the last date
of month.

Any help would be appreciated.

Step 1:
In B1, I enter =RIGHT(A1,2) & "/" & (LEFT(A1,LEN(A1)-2))
which gives me "12/2004"

Step 2:
In C1, I'm trying to use the formula

EndOfMonth = DateSerial(Year(B1),Month(B1)+1,0)
to return "12/31/2004" but I get the error #NAME?