hi there....
here's how i would calculate the time
1) make a list of all days you were *physically* present in canada
2) make a list of days you were outside canada on work provided the work was for a canadian company
add them up and if they go above 1095 days then...
for 2) you need additional documents like
a) each trip you made to US, you need a letter from your company manager saying what exactly that trip was for, which/where the client you visited and how long you stayed
b) if possible do get a letter from the client too stating that you were visiting them from so and so canadian company for work
c) make a photocopy of the pay stubs you received from your canadian company *during the duration of that particular US trip*
when you fill the citizenship form, they have a column for reason why you were out. in that column list the reason as working outside canada as a canadian employee and when you mail out the application, send all the documents from a) to c)
if you want, you can not send the documents and wait for them to question you about the documents, but i personally think you will save your time if you mail out the documents in the beginnin itself.
also, the total time you spent in US or outside on work, was it more than 3/4 months? if the days you spent in canada were less than 750, it's very tricky in that event.
as an aside, download the PR card form from the CIC site. browse that form and there you will notice that CIC is asking for specific documents from people who like you were outside canada for work. perhaps you can pick up tips on what more to attach with your application so CIC includes these days in canada for you.
feel free to ask any questions but please keep in mind this isn't a guaranteed way of getting the days included, just a precaution to make sure they consider your case. also, i would really ask you to call CIC and speak to an immigration agent and tell him/her that you possess all the above documents proving that u were working for canadian company in US, and check what they tell you.
hope that helps