public void StartTimer()
{
// Check if the last time period is still running. If so do nothing.
TimePeriod lastPeriod = TimePeriods.LastOrDefault();
if (lastPeriod != null && !lastPeriod.EndTime.HasValue)
{
return;
}
// Otherwise add a new timing period.
TimePeriods.Add(new TimePeriod {
StartTime = DateTime.Now
});
}