Count Unique Dates IF Corresponding Value NOT Null

I've come across a formula that I can't seem to get my mind fully around. I have a time-tracking sheet for employees that includes, among other fields, DATE and MILEAGE.

In my particular spreadsheet, DATE is from A2:A31 and MILEAGE is from G2:G31. The date column is set to the correct data type (date) and mileage is a general field and only has integers entered into it.

What I would like to do is count the number of unique dates on which mileage has been entered (field is not empty). If the total mileage for a day is less than 20, then do not count it. The reason for this is because an employee is entitled to be compensated for the total mileage of all service calls of a particular day minus 20 kilometers. If an employee, for whatever reason, enters say, 5km for the day, they should not be deducted 15km as would happen if the date was counted.

For example, if Jack traveled a total of 500km spread over 5 days, he is entitled to be compensated for 500 - (20 * 5) = 400km. The trick here is that if Joe has time billed on 6 unique days but only entered mileage on 5 of them, the formula still needs to return 5.

Here is what I have so far:

Code:
=SUM(G2:G31)-(20*SUM(IF(LEN(A2:A31),1/COUNTIF(A2:A31,A2:A31))))

This formula sums up the total mileage traveled and then subtracts 20 * number of unique days. The problems with it is that it still counts days on which no mileage was claimed (corresponding mileage field is empty). In addition, if the total accumulated mileage for a day is less than 20 then it is also still counted.