Change date format to use `as_json`

[jessieay]

• Hard-coded date string is more brittle
• Must call to_i on dates to control for difference in nanoseconds
• Fixes https://github.com/thoughtbot/ios-on-rails/issues/40

[srt32]

Looks good

[jessieay]

Merged 03427c324fd7175d308f02fbf609e5ef9c114118