Well, the message is rather stupid as there is no reason to pay any attention to the SPL dates. Each transaction has one date in an accounting system, not several, and SPL dates are redundant/extra information in an IIF file. The lack of logical thinking for whoever invented this message is astounding.
You can work around this and leave your file the way it is using these recommendations:
BRC QuickBooks 2019 IIF Import Changes and Recommendations
