Dealing with hours with no minutes when rounding DateTime to the 30 minutes

Question

I'm trying to emulate the way Google Calendar and Outlook handle time selection where all time entries are 30 minutes apart. I almost have every thing working with functional tests except for one thing. My problem is that I can't figure out how to ensure I'm always rounding the current date up to 30 minutes.

I'm using the following code:

protected DateTime GetStartTime()
{
    var spanTicks = TimeSpan.FromMinutes(30).Ticks;
    var ticks = (Time.Now.Ticks + spanTicks - 1) / spanTicks;

    return new DateTime(ticks * spanTicks);
}

This works for the majority of the cases except for 12:00am (where I would expect this to return 12:30am) and say 8:56am (where I would expect it to return 9:00am).

How can I fix this?