You needed to be more clear that you have left Canada 2 years as PR while living in US. This would make a different. Anytime left outside Canada would have to be make up for it inside Canada. You need to qualify as PR minimum of 2 years inside Canada in order to max out your pre-PR credit. Assuming you landed 2014, stayed until 2015, left Canada since and now 2017. You only got 1 year as PR to qualify. Using your pre-PR credit, you can only max 1 year towards citizenship going back 2012 assuming you file it this year. That leave you with 2 year total qualification. It's not enough. Since you need an extra 1 year as PR towards citizenship, you need to come back inside Canada for 1 year. But since you are depending on your pre-PR credit to qualify, you will need to stay even longer than 1 year to compensate for losing your half day credits. So by 2018, you got 2 years in as PR but you lost some 2012 credit. You need to compensate every day inside Canada for every 2 days lost in pre-PR days. So it is likely that you got around (guessing) 1.5 years to stay inside Canada to compensate your time outside Canada and lost pre-PR days.