Code

void Selection_Changed(object sender, EventArgs e)
{
      List<DateTime> newRanges = new List<DateTime>();
      for (int i = 0; i < calendar1.Selection.Ranges.Count; i += 2)
      {
            DateTime d1 = calendar1.Selection.Ranges[i];
            DateTime d2 = calendar1.Selection.Ranges[i + 1];

            TimeUnit unit = calendar1.ResourceViewSettings.BottomTimelineSettings.Unit;
            if (unit == TimeUnit.Month)
            {
                  DateTime newD1 = new DateTime(d1.Year, d1.Month, 1);
                  if (d1 != newD1)
                  {
                        newRanges.Add(newD1);
                        newRanges.Add(d1);
                  }

                  DateTime newD2 = new DateTime(d2.Year, d2.Month, 1);
                  newD2 = newD2.AddMonths(1);
                  if (d2 != newD2)
                  {
                        newRanges.Add(d2);
                        newRanges.Add(newD2);
                  }
            }
            else if (unit == TimeUnit.Year)
            {
                  DateTime newD1 = new DateTime(d1.Year, 1, 1);
                  if (d1 != newD1)
                  {
                        newRanges.Add(newD1);
                        newRanges.Add(d1);
                  }

                  DateTime newD2 = new DateTime(d2.Year, 1, 1);
                  if (d2 != newD2)
                  {
                        newRanges.Add(d2);
                        newRanges.Add(newD2);
                  }
            }
            else
            {
                  long unitTicks = 0;
                  switch (unit)
                  {

                        case TimeUnit.Second:
                              unitTicks = TimeSpan.TicksPerSecond;
                              break;

                        case TimeUnit.Minute:
                              unitTicks = TimeSpan.TicksPerMinute;
                              break;

                        case TimeUnit.Hour:
                              unitTicks = TimeSpan.TicksPerHour;
                              break;

                        case TimeUnit.Day:
                              unitTicks = TimeSpan.TicksPerDay;
                              break;

                        case TimeUnit.Week:
                              unitTicks = TimeSpan.TicksPerDay * 7;
                              break;

                  }

                  if (d1.Ticks % unitTicks != 0)
                  {
                        DateTime newD1 = d1 - new TimeSpan(d1.Ticks % unitTicks);
                        newRanges.Add(newD1);
                        newRanges.Add(d1);
                  }

                  if (d2.Ticks % unitTicks != 0)
                  {
                        DateTime newD2 = d2 + new TimeSpan(unitTicks - d2.Ticks % unitTicks);
                        newRanges.Add(d2);
                        newRanges.Add(newD2);
                  }
            }
      }

      for (int j = 0; j < newRanges.Count; j += 2)
            calendar1.Selection.Add(newRanges[j], newRanges[j + 1]);
}