i want to store buttn press time so that next time i won't have to press the button
I don't understand that. If you press the switch on September 13th, 2016 at 2:30AM, it will never be September 13th, 2016 at 2:30AM again, so knowing that you pressed the switch at September 13th, 2016 at 2:30AM is rather useless information.
However, there are functions (hour(), minute(), etc. that return VALUES that you can store in variables...