Date with timezone not displayed on calendar

[paulbriton]

When creating a new spreadsheet with a calendar column, if a date is formatted with time-zone (e.g. 2000-01-01T00:00:00+01:00) it won't be displayed by default but only after a double click on the cell. Works well without the time-zone and it seems related to #423.

Here is an example: https://jsfiddle.net/x7bqgfow/