Looks to me that you have chosen the wrong format. From your file, it seems you are trying to import period month, but the configuration you show is for date formatted property.
You should change the format of the property to Time Period, Month:
And the format in the upload should be the month and year without the dash: "Jan 20"
Then you should be able to map it and load it. There won't be any mapping options at all
But I also would suggest thinking about loading just the codes in the list and have a module with line items to load the data in. Or if your dimensions always have revenue per month, you might want to make a module dimensioned by "Dimension & Time: Month" and load the data directly in there.
You might want to look at this article that is really good for explaining the different types of loads and their impact on performance:
I am sure you are already aware of it but still let me say it, Don't load any data into the list especially when there is a time involved.
Now to your question: You can have any date format in the source file and the trick is to map it correctly and load it into Modules directly. I would recommend you go thru below best practice article here which coincidentally was written by me. Let me know if it doesn't help