If you don't report income that you should have reported, and it's more than 25% of the gross income shown on the return, or it's attributable to foreign financial assets and is more than $5,000, the time to assess tax is from the date you filed the return.
Or: For individual tax returns, call 1-800-829-1040, 7 AM - 7 PM Monday through Friday local time. The wait time to speak with a representative may be long. This option works best for less complex questions.