How do I change date based entries to numeric?

So I have some blog entries which are date based but I want them to be numeric so the site user can determine the order. This is what I did:

• Changed order: date to numeric in folder.yaml
• Renamed all entries: I changed the date to 1, 2, 3, etc.

Unfortunately the site and control panel stopped working. I get this error:

InvalidEntryTypeException in Entry.php line 197: Cannot get the date on an non-date based entry: [collections/blog/1.we-hebben-de-eerste-fiets-gedoneerd.md]

I tried clearing the cache, but this gives the same error. What am I missing?