Getting warning on create invoice

I get the following warning when attempting to create an invoice:
warning: getdate(): 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, you most likely misspelled the timezone identifier. We selected 'America/Los_Angeles' for 'PDT/-7.0/DST' instead in /xxx/modules/storm/storminvoice/storminvoice.module on line 917.