Cutoff flag per row: 3rd business day after service month end, but my dates are a mess

I’m a student in a small accounting internship and I got the classic “it’s easy, just do it in Excel” task: check cutoff on a monthly invoice dump. The export has Service Date (when the service happened) and Received Date (when AP actually got the invoice). The rule my manager wants is: if an invoice was recieved on or before the 3rd business day of the following month, it counts for the service month. If it’s received after that cutoff, flag it as Late for review. Business days are Mon to Fri, and we also have a simple holiday list on another tab (named range Holidays). I get the logic, but the file itself is giving me grey hair because the date columns are not consistent. Sometimes the export gives “2/5/2026”, sometimes “02/05/26”, and sometimes it’s literally “2026-02-05” as text. And of course some Received Dates are blank because they’re still waiting on the vendor. I can’t use Power Query on this laptop (IT locked it down), so it needs to be straight worksheet formulas.

Here’s a tiny sample of what the data looks like once it lands in the sheet (the real file is like 3k rows):

| Service Date | Received Date | Vendor |

| 1/29/2026 | 2/05/2026 | OfficeSupplyCo |

| 1/31/2026 | 2/06/2026 | Lab Rentals |

| 2/01/2026 | (blank) | Printing |

| 2/15/2026 | 2026-03-03 | Software |

What I want is two columns I can drag down: (1) the actual cutoff date for that service month, and (2) OK vs Late vs blank if there’s no Received Date. In my head it’s just EOMONTH + WORKDAY, but im getting a bunch of #VALUE and I can’t tell if it’s because some rows are text, or because Excel is “helpfully” treating 02/11/26 differently depending on my locale that day.

My current formual is basically:

=IF([@[Received Date]]<=WORKDAY(EOMONTH([@[Service Date]],0),3,Holidays),"OK","Late")

If I wrap DATEVALUE around the fields it fixes some rows and breaks others, and IFERROR just hides the chaos.

Is there a clean way to coerce both columns into real dates inside one formula (LET is fine), handle blanks gracefully, and still include the Holidays range? I’m trying to avoid adding 5 helper columns because this workbook is already turning into a monster and my manager wants it “simple”. Any best practice pattern for this kind of cutoff logic would save me.