Make tz support time conversions for other zones without a date change

[cameronsstone]

This change allows for conversion of a time in a timezone from your list (Warsaw in the sample) without a modified date. i.e. instead of putting

tz wars 0d 22:30

you can now just have

tz wars 22:30

I did some testing of the other functionality to check I hadn't broken it, but YMMV.