Just to confirm, the warning you receive looks like this:

The reason for the warning is that you're attempting to import a CSV that contains an invalid date/time format.

In this case the CSV you're importing contains columns called Start & End.  Both these columns contain data in this format dd/mm/yyyy.  The issue arises because you're importing into a date & time column, not just date time.  There are two solutions:

• Amend the data in the CSV file so that it includes a time and redo the import e.g. dd/mm/yyyy hh:mm.

• Set an appropriate Data Format setting in the New Field Mapping screen e.g. %d/%m/%Y.

This screenshot (click to enlarge) shows the difference between the default Data Formats for the Date and Date & time formats.

NOTE: If your date / date & time data is in a different / custom format you can specify it by typing the format in the Data Format field.