Your move to Japan in March 2018 will not affect your 2017 US income tax filing.
For 2018, you will need to remember to count your days in Japan for the Physical
Presence Test so that you will be able to claim a partial IRS
- Foreign Earned Income Exclusion (FEIE) for 2018.
If you pay Japanese income taxes on your Japanese income, you may also be able to take a Foreign Tax Credit for
taxes paid on income that is also taxed in the US.
When reporting your Japanese income, the Internal
Revenue Service has no official exchange rate. In general, use the exchange
rate prevailing (i.e., the spot rate) when you received the income. Please
refer to the following IRS links for more information about Foreign
Currency and Currency Exchange Rates and Yearly
Average Currency Exchange Rates
Additionally if you set up a foreign bank account in Japan, you may need to consider filing an FBAR related to all foreign financial accounts over $10,000 USD in aggregate.
