nshah1013,
The Now() function includes time of day and will often cause problems when used with date comparisons. It is better to just use the date() function in all cases unless you specifically want the time. In this particular case, Now() would be OK but it would NOT be OK if the condition were <=
Date() - DateField < 30
If it is possible for DateField to be a future date, you need to decide if you need to do anything different if the date is in the future.
In addition, if your field is named [date], rename it. MyDate, YourDate, whatever...but not [Date]. 'Date' is a reserved word, the name of a function, and it will ultimately come back to bite you.