Moment model may carry undesired precision

[bentrm]

I noticed that the moment object of a picker may not be "in sync" with the string representation. Whereas this may desirable at times I would like to argue that this is unintuitive.

I prepared a plunker to show what I mean by this.

Upon selecting a date in the format 'YYYY-MM-DD' I have the following result:

String date: 2017-06-27 
Moment date: "2017-06-27T09:03:15.641Z"

I would have expected an output as the following:

String date: 2017-06-27 
Moment date: "2017-06-26T22:00:00.000Z"

It's pretty much the output of moment('2017-06-27T09:03:15.641Z').startOf('date').toISOString() whereas date is the precision of the pickers format.

My current time zone offset (and summer time offset) is included in the calculation. I think this is a very good thing, as the timestamp represents my current notion of the beginning of the day at my location in a comparable moment object.

My suggestion would be, to offer an option to set the pickers moment object "precision" which comes down to the options of moment.startOf. This may be an optional attribute that only calls startOf on the underlying model if needed.

On a side note:

I'm not entirely sure, but I think this may be a solution to issue #163 as it would allow to set the output format independently making it easy to round the output of the format HH:mm to full hours via moment.startOf('hour').