为什么2005年的UTC时间与2006年的UTC时间不同?

TimeZoneInfo.ConvertTimeToUtc(new DateTime(2006, fromDate.Month, fromDate.Day),TimeZoneInfo.Local)
{7/24/2006 **4:00:00 AM**}
Date: {7/24/2006 12:00:00 AM}
Day: 24
DayOfWeek: Monday
DayOfYear: 205
Hour: 4
Kind: Utc
Millisecond: 0
Minute: 0
Month: 7
Second: 0
Ticks: 632893104000000000
TimeOfDay: {04:00:00}
Year: 2006
TimeZoneInfo.ConvertTimeToUtc(new DateTime(2005, fromDate.Month, fromDate.Day),TimeZoneInfo.Local)
{7/24/2005 **5:00:00 AM**}
Date: {7/24/2005 12:00:00 AM}
Day: 24
DayOfWeek: Sunday
DayOfYear: 205
Hour: 5
Kind: Utc
Millisecond: 0
Minute: 0
Month: 7
Second: 0
Ticks: 632577780000000000
TimeOfDay: {05:00:00}
Year: 2005