Cannot set the correct WeekDay.Shift1.Start time

I'm encountering a issue in MSProject 2010 while programming MSProject Add-in Plugin

MSProject.WeekDay TimePeriod;

TimePeriod.Shift1.Start = GetStartTime();

GetStartTime() returned a DateTime Object with value '2012/7/31 8:00:00', after executing TimePeriod.Shift1.Start = GetStartTime(); the value of TimePeriod.Shift1.Start was '7:58'. I tried different times, such as '2012/7/31 7:58:00','2012/7/31 7:59:00','2012/7/31 8:01:00', but the result of TimePeriod.Shift1.Start sometimes matched the set value, sometimes not. Only occured when the name of TimePerid was 'Mon', other times('Tue', 'Wed', 'Thu' and 'Fri') were correct.

This never happened in MSProject 2007.

Is this a bug of MSProject 2010?

Thanks.