Timestamp error

I am installing this plugin for the first time with a new Payeezy account. When testing, and I submit the form, I get a flash of the following error, then a hosted page that says checkout is ended due to technical error. Here is the error I’m getting:

Warning: strtotime(): It is not safe to rely on the system’s timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, y….. Please set the date.timezone to select your timezone in /pay-rec.php on line 55 and 56.