Did you enable auto-enter options during the import? What happens if you remove the auto enter field option?
You may have something to report as a bug in Report an Issue--though this doesn't match any other reports that I've read and I would think that failure to import a date from Excel would have produced such a report a long time ago.
I'd not use an import file for this. I'd just paste the return separated list int a global text field in FileMaker and set up a script that first uses the pasted list to find the records (can be a simple Go TO Related records step) and then uses replace field contents to update the dates.
I'll try your suggestion (haven't done anything like this before, so I may have another question), and I forgot to say, I hadn't tried clearing the auto enter option, but I just did and the import worked. Why? Something about 13?
My guess is that the data is being imported and then the auto-enter option clears the imported data.
This won't happen during an import unless:
"do not replace existing value..." is cleared
The "enable auto enter options" check box has been selected in the import records settings.
The key trick behind my suggestion is that if you put a return separated list of values into a text field and use that field as a match field in a relationship, records on the other side of the relationship will be related records if their match field matches any one of the listed values.
So if I put:
into such a match field. Records on the other side of the relationship will link to this record if their match field is apple OR kiwi or orange....
That allows you to basically do what you need in two steps:
Go To Related records
Replace Field Contents