The U.S. government will exhaust its borrowing authority between mid-February and early March if Congress doesn’t raise the debt ceiling, outgoing Treasury Secretary Timothy Geithner told Congress in a letter today.

The nation nudged against the $16.4 trillion debt ceiling just before the New Year. Geithner has since been using accounting maneuvers — known as “extraordinary measures” — to keep the nation from missing its debt payments and other bills. …

Geithner warned that the extraordinary measures will be exhausted “between mid-February and early March of this year” — and that lawmakers must raise the debt ceiling if they want to keep the nation from defaulting.

Geithner said it’s difficult to pinpoint the exact date the nation faces default because of tax season.