The continental United States spans four standard time zones. The East Coast operates on Eastern Time, while the West Coast uses Pacific Time, creating a three-hour difference: when it's noon on the East Coast, it's 9:00 AM on the West Coast.
During daylight saving time (roughly March through November), both coasts shift forward one hour, so the three-hour gap remains constant. The Mountain Time Zone (one hour behind Eastern) and Central Time Zone (two hours behind Eastern) fall between these coasts.
This time difference exists because the Earth rotates west to east, so the sun rises earlier in the east. The continental U.S. is wide enough—roughly 2,800 miles from coast to coast—that significant sun position differences occur. Time zones were established to keep local solar time roughly aligned with clock time, preventing situations where noon occurs at 3:00 PM or breakfast happens in darkness.
When coordinating calls or events between coasts, accounting for the three-hour gap is essential. Morning meetings on the East Coast occur in early morning or overnight on the West Coast, which is why many national broadcasts and meetings are scheduled around this constraint.