在Ruby中，我如何只在一天中的特定时间内为一个Datetime增加15分钟?

I'm a full time SQL Server database developer, but I'm looking to start utilizing Ruby for certain management tasks. Also, I'm trying to take my Rails skills to the next level. I'm great with SQL, but weak in programming languages (I'm working hard to fix that!). Here is my question:I'm a full time SQL Server database developer,