There was originally a GEP of 3 months (October through December) in every second
year beginning with 1967.
The first GEP began 10/1/67 and should have ended 12/31/67, but the Congress extended
it through 3/31/68 (actually, 4/1/68, since 3/31/ 68 was a Sunday). Thus, this GEP
lasted 6 months. After 1968, the GEP occurred each January through March, except as
noted in 3. below.