Underscores work well, which results in "2013-12-16 10_22_57Z". I can still somfortably read it, at least. Hyphen-minus would lead to too easy confusion with t

Underscores work well, which results in "2013-12-16 10_22_57Z". I can still somfortably read it, at least. Hyphen-minus would lead to too easy confusion with the date part. Fixed-width fields can be hard to read.