Make TimeItem eligible to render negative time offsets

[fatzopilot]

In certain situations, it is convenient to store time offsets as dates (using new Date(0) as a reference.), e.g. to hold timezone offsets.
It would be nice to have TimeItem render negative offsets appropriately such that a user can input something like -04:00. Currently, this reverts back to 00:00.

[fatzopilot]

Just noticed that I would have to support timespans longer than 24 hs as well to be useful in this context.
Maybe a specific TimeSpanItem would make more sense here.