Example. A taxpayer filed a return for the calendar year 1973 on April 15, 1974; the notice of deficiency was mailed to him (at an address within the United States) on April 15, 1977; and he filed a petition with the Tax Court on July 14, 1977. The decision of the Tax Court became final on November 6, 1978. The running of the period of limitation for assessment is suspended from April 15, 1977, to January 5, 1979, which date is 60 days after the date (November 6, 1978), on which the decision became final. If in this example the taxpayer had failed to file a petition with the Tax Court, the running of the period of limitation for assessment would then be suspended from April 15, 1977 (the date of notice), to September 12, 1977 (that is, for the 90-day period in which he could file a petition with the Tax Court, and for 60 days thereafter).
26 C.F.R. §301.6503(a)-1