Avoiding taxable gain by living in property 2 of last 5 years

[James N.]

Hello BP,

I have a condo I lived in for about 2 1/2 years as my primary residence, then I leased it out for about 2 1/2 years. I'm looking to sell and avoid taxable gain. I'm trying to determine exactly what day I have to sell by because I know it had to be my primary 2 of the last 5yrs from the sell date. I don't remember the exact day I moved out. It was around April 2016, then it was listed on the market in May 2016. A lease was signed in August 2016.  

So would the day I listed it on the market be considered the day it was no longer my primary residence? If so, how would they confirm the exact move out date since I can't even remember?

How do they determine a property is no longer your primary residence for tax purposes?