Don't unconditionally put 'Z' suffix at the end of dates.

ISO-8601 / http://www.w3.org/TR/NOTE-datetime mentions:

  This profile defines two ways of handling time zone offsets:

  1. Times are expressed in UTC (Coordinated Universal Time), with a
     special UTC designator ("Z").

  2. Times are expressed in local time, together with a time zone
     offset in hours and minutes. A time zone offset of "+hh:mm"
     indicates that the date/time uses a local time zone which is "hh"
     hours and "mm" minutes ahead of UTC. A time zone offset of
     "-hh:mm" indicates that the date/time uses a local time zone
     which is "hh" hours and "mm" minutes behind UTC.

  A standard referencing this profile should permit one or both of
  these ways of handling time zone offsets.  "

The 'Z' character will be part of the data string already if needed,
adding a second one causes various tools to behave badly.