"The dates all seem to be interpreted by Quicken as day/month/year format ....".
The U.S. version of Quicken interprets QIF file dates as being in month/day/year format.
But when two digit years are used in the QIF file, the creator of the QIF file must take into account the change of century (whether year "19" is 2019 or 1919).
If the source of the QIF file is not formatting the two-digit year dates correctly, the best way to correct that would be for the source of your QIF file to offer to download QIF file dates in the format mm/dd/ccyy (or to format the two digit years correctly to distinguish the 20th century from the 21st century).
Otherwise, you may be able to use the free Excel Add-in, xl2qif (Google will find it) to import your QIF file into Excel, then export the Excel data to a new QIF file, having xl2qif change the date format to meet with Quicken's expectations, then import that new QIF file into Quicken.
If you want to see the effect of the formatting of two-digit years in QIF files, try this:
- Create a New (test) Quicken file
- In that New (test) Quicken file, create a Cash account
- Copy the data below into Notepad and save as a .QIF file
- Import that QIF file into your New (test) Quicken file Cash account
- Carefully note the Transaction dates in the imported Quicken transactions
!Type:Bank
D01/15'19
PMemo. QIF Date = 01/15'19
T-110.00
^
D01/23/19
PMemo. QIF Date = 01/23/19
T-225.00