There are two ways to send dates across the wire as text between different systems:

1) ISO 8601

2) A world of hurt

Source: experience. Not using the standard invites things to break, but only on a some machines, and only on or after the 13th of the month. Of course the data for the first few days of the month will be present, just wrong.

And sadly there are still things that insist on using the second option. RFC 2822 used in MIME (historical oddity because it pre-dates ISO 8601), UNIX timestamps as either seconds or milliseconds (at work we have one instance where numbers > 32 bit are serialized as a hex string in JSON, which includes such a millisecond-UNIX-epoch timestamp), etc.

Slightly related insanity: Using localtime (i.e. not UTC) for log timestamps. And then having fun once a year dissecting the one hour that has twice as many messages.

And always - always - always in UTC.

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

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.

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

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

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