Hey congrats on you and your girlfriends' decision. All the best to both of your careers and relationship.
btw, I used prepared US returns for expats in HK, and other asian countries. The 140 days applys to non-US citizens because the thing is if you are non-US citizens, your income will be taxed if you worked in the US for 140 or more days for that year.
As for US citizens, there is the automatic extention of time for filing. (You may even keep on applying as long as it's reasonable in terms of your situation) To play safe, you can choose to pay the similar amout (Estimated Tax Payments from 90% to 110% depending on your situation) of $ to the IRS first and submit to return later. (if you don't mind losing the interests you would have earned in the bank or taking it out from investments). The IRS will pay you back certain amount of interest if it happens that you don't owe them the amount you paid.
Also, as you work and live overseas, you should claim your expenses for housing and foreign earned income (both are limited to certain amounts and depend on your aggregated gross income) with form 2555.
[Having said that, I do not take any responsibilities for the above information. Consult your accountant for professional advice before filing your tax return.]
Anyhow, the next deadline is coming soon. Good luck!