Allow users to enter dates manually OR use the calendar widget to enter the date. TemplateWizard currently enforces use of the date picker to pick dates and inserts them in ISO format. The widget should be switched to be the one used by UploadWizard and Special:Block -
While testing this extension with Template:Singles on the English Wikipedia, the form forced me to insert dates in ISO format because of parameters being classified as "date" in the template's TemplateData. I suspect that for various reasons there are approximately zero templates on the English Wikipedia where single-parameter dates should be entered in ISO format, even if it may be useful to do so on other wikis like the Chinese Wikipedia.
Even if this is a documentation error (I wrote the TemplateData for that template and I don't remember if date fields are always supposed to be ISO), the extension could be made locally configurable to allow users to enter any date instead.