change determination of date string for mindate and maxdate

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #1361, #1360
| License       | MIT
| Doc PR        | -

@ansch - please take a look at this. I think this is what you needed and also doesn't set `$mindateString` to null