Finding zero crossings of sine wave and its maximas

Hello All,

I need a real live saver here . In the excel sheet attached (plot of voltage versus time), I have values from a sine wave and I want to detect automatically the zero crossings in column B and register the average of time stamps (from column A) into column D (Zero crossings). I have done the first one manually, but how to automate for the complete series?

Also I need to register the times (from column A) at which voltage (column B) goes above certain threshold say 7900.

Any advice please????