pfSense

Possible candidate.
http://morecavalier.com/index.php?whom=Apps%2FUnix+timestamp+converter

That is an EXCELLENT converter, probably the BEST and ONLY one which allows for free-form (eg. 11/9/2011 12:15:59) input of time string, and can also take into account for both UTC and/or GMT timezones. All others requires separate fields for input and only has code for UTC times.

Would still be nice to have a JS Date Picker in the GUI end so people don't have to type it out. Clicking around a calendar is a lot easier. :-)

Okay we have a datepicker now:
https://github.com/bsdperimeter/pfsense/commit/ce49a7c9f2781bda704e4e561026b5e445ab1fcb
Hope the jQuery UI theme I selected matches pfSense closely enough. :-)

I hope the date picker will have the ability to accurately "pick" hours, minutes, and seconds rather than rounding to nearest hour/minute or so.

Also I like how some apps on smartphones/tablets allows for BOTH freeform/keyboard AND calendar/GUI picking.

Just curious, during Daylight Savings Time Change, does pfSense's RRD graphs discard or keeps, the added OR subtracted hour of data?

I can't speak for daylight saving time handling but the rest of your needs are fit by this.

http://trentrichardson.com/examples/timepicker/

See that for a demo. It's pretty customizable.