如何管理php中的日期序列以显示下一个日期剩余的天数？

I have a start date like 07/18/2014, a end date like 07/24/2014, and a period date: every 2 days. So every 2 days starting by 07/18 i warn the user that something have to do, if is not the right day i warn the user about how left days remaining to the next warning. How can i do? And if add also time and a period of every 2 hours?I have a start date like 07/18/2014, a end date