Why is Orthodox Christmas on January 7?

Why is the Orthodox Christmas on January 7, almost two weeks after ours?

A few decades before Jesus was born, Julius Caesar reformed the Roman calendar by adding one day every four years.  This worked well for a long time, keeping the calendar in line with the seasons.  

But in the 1600s, after the Reformation, the calendar was noticeably off, so Pope Gregory  refined the Julian calendar by making century years not leap years unless they were divisible by 400 and resetting the calendar to put it back in line with the seasons. 

Julian Calendar
4 years x 365 days/year	1460
Add 1 day for leap year	1461
No. of days per year	365.25
Changes made by Gregorian Calendar
No. of days in 400 years	146100
Minus 1 day for each century year not divisible by 400	146097
Average # of days per year for 400 years	365.2425

Every century year, except when the century year is divisible by 400, the Julian calendar differs from the Gregorian calendar by one more day. For instance, when England adopted the Gregorian Calendar, they were off by 11 days.  Now the difference is 13 days, so December 25 on the Julian Calendar is January 7 on ours.