Hanvyj, my first thought was of times I've had a problem like this, where I'm doing MATCH or VLOOKUP on values that appear numeric, but in either the source or the target worksheet they're actually character strings. But you say you did <source>=<target> and got True back, so that's not the problem here.

It also occurred to me that you're searching for 09:00 (0.375) when the actual value is <date> 09:00 (n.375). Again, that doesn't seem like it can be the problem here.

A rounding error? Nah, shouldn't be that.

I just copied a date value to three other cells and formatted them as General, yyyy-mm-dd and yyyy-mm-dd hh:mm; but both the equality test and the MATCH function agrees that they're all equal.

I dunno, I have to agree with Pecoflyer; I need to see it.