The reason we used the DateFormat factory constructors, instead of just allowing the developer to specifying an arbitrary format string, was because it eliminated the possibility of a runtime error due to a bad format string. I agree that it's reasonable to allow developers to specify an arbitrary format string - as an alternative - however I don't think we should stop recognizing the ones that correspond to DateFormat factory constructors. I assume that most apps, most of the time, will be able to use the formats for which there is a factory constructor.

We want to avoid breaking anything and to encourage developers to continue to use the standard formats when possible. For example you could introduce a placeholder attribute like isCustomFormat, ("isCustomDateFormat": true) which would have to be present if format's value was an arbitrary DateFormat string. There are certainly more convenient ways to express this, but we'd prefer that developers only use custom formats when that's necessary. And when they do, they should do so explicitly.

Sorry about all of the direction; hope this makes sense.