You can usually use VBA to easily spot the errors, so:
IIf([START DATE] < DateSerial(Year(Date),[WinterStartMonth],[WinterStartDay]) _
And [START DATE] > DateSerial(Year(Date),[SummerStartMonth],[SummerStartDay]), _
DateAdd("d", [WinterInspectionDropDead], [START DATE]), DateAdd("d", _
[SummerInspectionDropDead], [START DATE]))
Just remove the line break characters for SQL:
IIf([START DATE] < DateSerial(Year(Date), [WinterStartMonth], [WinterStartDay])
And [START DATE] > DateSerial(Year(Date),[SummerStartMonth], [SummerStartDay]),
DateAdd("d", [WinterInspectionDropDead], [START DATE]),
DateAdd("d", [SummerInspectionDropDead], [START DATE])) As Result
Or probably better:
IIf([START DATE] < DateSerial(Year(Date), [WinterStartMonth], [WinterStartDay])
And [START DATE] > DateSerial(Year(Date),[SummerStartMonth],[SummerStartDay]),
[START DATE] + [WinterInspectionDropDead],
[START DATE] + [SummerInspectionDropDead])
As Result