Calculating mean reversion half life

I am trying to calculate the half life to mean reversion of a time series. This is what I have done so far. Am i barking up the wrong tree???

Working in excel I have the daily closes of a share price from which i have calculated the change from the previous day of the share price.
I have used the slope function to give me a gradient using Y as the change in price from the previous day and X as the share price.
i have then taken this number (z) and done the following calculation:-

Half-life = LN(2)/Z OR IS THE EQUATION Half-life = LOG(2)/Z OR MORE WORRYINGLY IS IT NONE OF THIS AT ALL!!!!!!!!!
why is the answer negative? It cant take a negative time to mean revert?? or do i just take the absolute value???????
Any help appreciated