I'm not quite sure what you mean by 'counting back from March 2018'. If things have not changed lately, what I know is that you have to be in canada in total of 3 years (3*365 days) in the past four years(4*365 days) to be eligible to apply. If you were in canada within that 4 years and did not have PR, those days will also be counted. For example, you were in canada for 2 years in some other visa before you get your PR, you can apply for citizenship after 1 year of getting PR.