Whilst I'm not sure how it would work for your
per diems, I can heartily recommend Skylog Pro which covers duty limits of all kinds and has a natty night hours calculator function. It also features a world airport database, and can generate more report formats than you can shake a stick at!
Skylog homepage
Best of luck with whichever you choose. (It's worth bearing in mind that computerised log book print-outs can be difficult to present at job interviews).