[hessenwolf]

It's annoys me that the time format with colons means I cannot use it for file names. What do you use instead?

[ygra]

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.

[SideburnsOfDoom]

Some other innocuous separator character such as underscore, or none, just fixed lengths for the numeric fields. But always, yyyy mm dd order.

[Semiapies]

I just use HHMM at the end (or HHMMSS if you need that precision).

Ex: 2013-12-16_1741.txt and the like.