A date field stores a date as an integer that counts the number of days from 12/31/0000 to the date displayed in the field. So comparing the value to 30 will not produce the desired result as no date that you enter into this field will have a value that small.
Work Orders::Date of Work Order + 30 > Get ( CurrentDate )
is more in line with what you want here, though this would need to be defined in an unstored calculation field.
Note that you can use a script like this to find all such records in work orders without using a calculation Field:
Enter Find mode [ ]
Set field [Work Orders::Date of Work Order ; ">" & Get ( CurrentDate ) - 30 ]
Set Error Capture [on]
Peform Find [ ]
This will be faster than searching for a value in an unstored calculation field
For more examples of scripted finds of this type, see: Scripted Find Examples