To round down to the second:
dateTime.AddTicks(-dateTime.Ticks % TimeSpan.TicksPerSecond)
Replace with TicksPerMinute
to round down to the minute.
If your code is performance sensitive, be cautious about
new DateTime(date.Year, date.Month, date.Day, date.Hour, date.Minute, date.Second)
My app was spending 12% of CPU time in System.DateTime.GetDatePart.