sec 102: tax exemption on sale of Primary residence

[Eric Lo]

Hi BP community! There are a few questions I cannot figure out a practical answer regarding sec 121 and the rule of "2 of the last 5" regarding marriage (to achieve the full $500k exemption). IRS states you must and your spouse must have co-habitated in order to receive the exemption, although it clarifies that you didn't need to be married at the time of co-habitation if you married after. My questions with regards to the practical enforcement by the IRS of this rule:

1) Does co-habitation have to occur during both years of the "2 of 5"? And what happens if its partial co-habitation for one of the two years?

2) What determines co-habitation? For example does it have to be a demonstrated primary residence or would say staying on all weekends meet the bar?

3) Lastly (and perhaps most simple/stupid), when does the 5 year rule start/end and is it based off of 5x365 days of the last day that the home was primary, or is it just in full calendar/tax years (ex. if last day was January 1st of 2010, the exemption would apply as long as the sale occurred before Dec 31 2014)