Cannot assign calculated value to user field in Rules
I could not figure out why the result variable was not available; my solution was to change the order of the actions to:

After the user clicks save button, rule intercepts:


get the value of author's "days left" field and multiply with 86400 (convert days to seconds)
get the duration of content and subtract it from the previous value
divide the result by 86400 (convert it back to days from seconds) 
convert the result to integer
subtract one from this result (this turned out to be necessary, perhaps because of the rounding off during the type conversion)
assign the result to content author's "days left" field.


I do not like having to make two type conversions but this is a working solution.