[Click on pictures to enlarge]

The correct date format that Odoo accepts is YYYY-MM-DD. The way I know this is because I exported an existing bank statement in "Import-compatible export" in CSV and looked at how it was made. You export in the list view after clicking the checkbox of the records you want to export. Here I exported 1 bank statement that contains 4 lines. The export window looks like below and I exported the bank statement date and the date of each line:

If I open the CSV in Excel, I see this:

Be careful with spreadsheet editors, they tend to display dates differently than how they are stored. This format looks weird, so I decided to open the csv file in a text editor:

You see that the date format is YYYY-MM-DD.

Now, I can reuse this template to re-import a new bank statement in Odoo. It helps to have the correct column headers. Note that I only did this test with a few fields; more fields can be added. I changed the data as follows:

Back in the list view of bank statements of Odoo, I clicked on the Import button and selected my file. All the fields are recognized automatically.

When I validate, a green message indicates everything is correct, and I then click on Import.
My new bank statement appears then:

Note: you don't necessarily have to use a text editor to make a change in the CSV or reformat a date. I did it here to warn about the fact that certain spreadsheet editors display something different than the encoded data. It is still possible to use a spreadsheet editor like LibreOffice, Excel, or Google spreadsheet. You'll have to play with the "Format Cells" feature.