DateTime.fromFormat is not able to correctly parse the result of DateTime.toFormat()

The following code results in an invalid DateTime:

const x = new Date();
DateTime.fromFormat(DateTime.fromJSDate(x).toFormat('D T'), 'D T')

error on the resulting DateTime object:

invalid: {…}
​​explanation: "you specified 20 (of type number) as a month, which is invalid"
​reason: "unit out of range"

I’m using firefox 84.0.2 on windows 10, lang in browser set to en-GB, windows region format set to Polish, windows display language set to en-GB, windows apps and websites land set to en-US.

[dan-overton]

I think if I was expecting US formatted dates from a backend, I’d expect to have to specify en-US explicitly in fromFormat. That is, I’d find having to do that that less unintuitive than this bug.

If that’s the expected behaviour though, I guess this one can be closed?

[spooky]

I confirm that passing the locale explicitly in the options resolves this issue. The reason I reported it though is that when running on defaults for both fromFormat and toFormat, I’d expect this to use the same locale whereas it seems that’s not the case.