**List of steps to reproduce**
```
a = new mw.widgets.DateInputWidget()
a.setValue('25/05/2002')
console.log(a.getValue())
a.getValidity().then(() => console.log(true), () => console.log(false))
// "2002-05-02"
// true
```
```
a = new mw.widgets.DateInputWidget({inputFormat: 'DD/MM/YYYY'})
a.setValue('25/05/2002')
console.log(a.getValue())
a.getValidity().then(() => console.log(true), () => console.log(false))
// "2002-05-02"
// false
```
**What happens?**:
date is invalid and `mw.widgets.DateInputWidget.prototype.setValidityFlag()` sets the flag to invalid; hence a red border appears around the html element
**What should have happened instead?**:
date is valid and html input element does not have a red border
**Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.**:
mw 1.38; i pasted the code into the dev console on https://www.mediawiki.org
`mw.widgets.DateInputWidget.prototype.isValidDate` creates a moment object with the custom date input format verifiying it against the internal date value `yyyy-mm-dd`; could the internal date format be used when comparing it against the internal date value?