Max date per month

I get multiple invoices, each containing a column of service dates. These can come in with some weirdly formatted dates, which I have to convert to a TxServDt column. My formula of choice (works best on most invoices) is this:

=IFERROR(INT(TRIM(REPLACE($A3,7,0,","))),1*$A3)

Now there's a new wrinkle: instead of the equivalent date of what the vendor supplies, I instead need the last date in the month (not "last day of the month", but "last date in the column for the month in question", so max date per month). See the example attached. Column A is what the vendor provided, column B is what my formula supplies, column C is the result I need. Note that we have May, June and July dates, so what I need is (in this example), 5/30, 6/30, and 7/28.

How, for my TxServDt column, can I both convert the date AND make sure it's the max date for that month?

Also, related, my formulas are delivered through a macro. Each vendor puts their date column in a different area, and calls it something different, and formats it differently. So a formula is best because if my macro chooses the wrong column (there are actually multiple different date columns, so it's a guessing game for my macro) I can re-aim the formula to the right column.

Any help is appreciated.