This is not a "stupid question" at all; and we will be happy to answer it for you.
For purposes of qualifying for the Foreign Earned Income Exclusion, as a United States taxpayer, you must meet at least one of two different tests. They are known as the Bona Fide Residence Test and the Physical Presence Test, and are explained on Pages 1, 2, and 3 of the instructions for IRS Form 2555, which can be read here:
https://www.irs.gov/pub/irs-pdf/i2555.pdf
Referring to the 330 days you mention in your question, this would be asking about the Physical Presence Test, to which the IRS has the following to say:
"To meet this test, you must be a U.S. citizen or resident alien who is physically present in a foreign country, or countries, for at least 330 full days during any period of 12 months in a row. A full day means the 24-hour period that starts at midnight. To figure 330 full days, add all separate periods you were present in a foreign country during the 12-month period. The 330 full days can be interrupted by periods when you are traveling over international waters or are otherwise not in a foreign country. See IRS Publication 54 for more information and examples.
The 12-month period on which the physical presence test is based must include 366 days, part of which must be in 2016. The dates may begin or end in a calendar year other than 2016."
So, what this means in practical terms for your specific situation, if you lived and worked abroad for only 3-months (90-days or so), then you would not be able to qualify under either of the Foreign Earned Income Exclusion tests, and you will therefore have to pay full regular US income taxes on your overseas earnings.
However, if you worked abroad for 3-months, but then continued to reside outside of the United States for the other 8-months required to meet the 330 day mandate (a.k.a. 11-months out of 12-months), and were not working during that time, then you could qualify for the Foreign Earned Income Exclusion under the Physical Presence Test, which would exclude much (or all) of your foreign income from US taxation. If this applies to you, then you would benefit by filling out IRS Form 2555.
Does that reasoning and explanation make better sense to you now?
Hopefully it does, and thank you for asking this important question.
