When performing a data upload using an xlsx file and you are uploading data to a date field, there may be issues with the upload.
For example if you have a custom field eg comp_dateTest added to the Company entity and you are uploading an xlsx that has data for this field you may encounter an error if the field is set to a date field in the xlsx file. 

A data upload will not work correctly due to issues with Date fields. The following error will be generated:

Warning	Invalid DateTime Value

This is a known bug and has should be resolved in 7.3c.

More information:
The simplest option as a workaround to avoid this issue is to save the xlsx file as a .csv.
If you then use the .csv for the data upload the issue should no longer occur.