SE4.md: Switch to ISO8601.

Switch to ISO8601 (yyyy-MM-ddTHH:mm:ss.SSSZZ) format from
java.util.Date#toString()'s amazing 'EEE MMM dd HH:mm:ss zzz yyyy'
format.

This still leaves one instance which isn't ISO8601, which is status's
built_when, which is 'yyyyMMdd-HHmm'.

Also, switch the type to DateTime.