All date properties of the various response models are formatted in compliance with ISO-8601. In general the format is “Year-Month-Day’T”Hour-Minutes-Seconds-Ten MIllionths of Seconds-Offset from UTC in minutes”. The ‘T’ character acts as a separator between the date and time part of the value.

Example:

"2015-04-10T00:51:18.0000000+00:00"

For Formz that contain a Date, Time, or DateTime field, the value of the field will contain a value property formatted as shown above, along with a displayValue property, containing the text that was displayed on the form.

Example:

"Date 1": {
    "value": "2015-04-09T00:00:00.0000000+00:00",
    "displayValue": "4/9/2015",
    "id": "...",
    "name": "Date 1",
    "type": "Date"
    }

For Dates, the Time section will be set to midnight UTC and for Time fields, only the portion of the date after the T character will be returned. When submitting values, make sure they are encoded as above in order to avoid validation errors.
Example:

"Date 1": {
    "value": "2015-04-09T00:00:00.0000000+00:00",
    "name": "Date 1",
    "type": "Date"
    }

We do not currently preserve the timezone of a date, datetime or time field entered on a form. If a user enters "4/9/2015" on their form, the api will return the value "2015-04-09T00:00:00.0000000+00:00" even if it was entered from a different timezone.

See https://msdn.microsoft.com/en-us/library/az4se3k1%28v=vs.110%29.aspx#Roundtrip for more info on the format used.