If you are a US citizen or green card holder you can exclude income from your federal return using the foreign earned income exclusion and/or the foreign tax credit. You cannot exclude income due to the treaty.
If you did not file PA taxes in 2017 and only moved to PA in December 2018 , you should not be paying tax on income you earned in Japan.
Part-Year Residents of Pennsylvania. Part-year residents are subject to PA personal income tax on all income earned, received and realized from all sources when residents of PA. Part-year residents are also subject to PA tax on income earned, received and realized from sources within PA when they were not a PA residents. Part-year residents are not subject to PA tax on ordinary interest, dividends, gains, intangible property or gambling and lottery winnings from PA sources while a nonresident. Some exceptions apply; see below for additional detail.
