"Taxable Period" For Correction of a funding Deficiency

[Guest merlin]

A calendar year plan has a funding deficiency as of 12/31/03. The 10% 4971a excise tax has been paid. What is the ending date of the taxable period for correction of the deficiency, thereby preventing impositon of the 4971b 100% excise tax?

[Guest merlin]

Looks like 90 days from the date they're officially notified by IRS of the deficiency, per 4963e1. But I can't see how to get there from 4971c3.