If I was on short term disability for 5 weeks during the year, does that mean that my 2017 return does not cover a full 12 month period? I'm trying to figure out if I can avoid the underpayment penalty.

[DDollar]

No, it does not mean that your return does not cover a 12 month period.  That would apply in a situation such as you if you were a non-resident alien part of the year, and a resident alien part of the year.