foo must be an RFC 3339 date-time

The content of this element MUST conform to the "date-time" production as defined in RFC 3339. In addition, an uppercase "T" character MUST be used to separate date and time, and an uppercase "Z" character MUST be present in the absence of a numeric time zone offset.

Change the date-time format to comply with RFC 3339. Here are examples of valid RFC 3339 date-times:

2002-10-02T10:00:00-05:00

2002-10-02T15:00:00Z

2002-10-02T15:00:00.05Z

Common errors include using a space to separate the data and time, or omitting the time zone information.

Other potential errors include individual fields being out of range, for example a month of 13, or a day of 31 in a month that only contains 30 days.

Note that RFC 3339 is more strict than the W3C Date and Time Format. In particular, all portions of the date and time must be present, with the exception of the fractional portions of the second, which may be omitted.