Their are two time periods that are required to be covered on the N-400 application. Either five years in the US prior to filing the N-400 or the the entire period you have been a lawful permanent resident of the US, whichever is longer. For instance if you spent 4 years in non immigrant status and married a US citizen and you are now seeking naturalization based on being married to a US citizen, you may be a permanent resident for three years but need to include trips going back 5 years to include the last two years in non immigrant status.
The date you entered the US with the immigrant visa.
J Charles Ferrari
Eng & Nishimura
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The N-400 form requests information for all trips made outside the US since becoming a lawful permanent resident. I recommend that you consult with a qualified US Immigration & Nationality lawyer to seek additional guidance.
Date you obtained lawful permanent residence (LPR) in the US. The USCIS needs to determine if you've ever abandoned your LPR status and if you're eligible for US citizenship based on the residence requirement.