Just to elaborate on DH’s last post:
Sha’ah Z’manios would tell you the length of your 6-7pm “hour.”
Your position within your time zone would tell you when that “hour” starts (regardless of how long it is).
The position in your time zone explains why some places start Shabbos earlier (and have sof z’man krias sh’ma, etc.) than others in the same time zone. Since (just to give an example) New York is further east than Scranton PA, sunset comes to New York before it comes to Scranton. So (just to pick numbers out of thin air) if Shabbos begins at 5:30pm in New York, it might begin at 5:47pm in Scranton. So too, the midpoint between noon and midnight (which is what the 6:00 hour is) would be earlier in New York than in Scranton.
To further understand why this is so, you might want to read a bit on the history of time zones:
http://en.wikipedia.org/wiki/Time_zone#History
The Wolf