Run user defined function for each row matching criteria of array formula if statement - excel

Question

I made a custom function in Excel VBA:
Option Explicit
Function Units(budgetYear As Integer, birthday As Date, serviceStart As Date, serviceEnd As Date, isMarried As Boolean, isServingAbroad As Boolean) As Double
' in this function all dates are converted to the first day of the date's month to make calculations easier
Dim units As Double: units = 0
' service end date cannot be earlier than service start
If serviceEnd < serviceStart Then
Units = units
Exit Function
End If
Dim firstMonthOfYear As Date: firstMonthOfYear = DateSerial(budgetYear, 1, 1)
Dim lastMonthOfYear As Date: lastMonthOfYear = DateSerial(budgetYear, 12, 1)
Dim newServiceStart As Date: newServiceStart = FirstOfTheMonth(serviceStart)
Dim newServiceEnd As Date: newServiceEnd = FirstOfTheMonth(serviceEnd)
Dim eighteenthBirthday As Date: eighteenthBirthday = DateSerial(year(birthday) + 18, Month(birthday), 1)
Dim thisYearServiceStart As Date: thisYearServiceStart = WorksheetFunction.Max(newServiceStart, firstMonthOfYear)
Dim thisYearServiceEnd As Date: thisYearServiceEnd = WorksheetFunction.Min(newServiceEnd, lastMonthOfYear)
Dim serviceMonthsThisYear As Integer: serviceMonthsThisYear = (Month(thisYearServiceEnd) - Month(thisYearServiceStart)) + 1
' unit multipliers
Dim serviceTypeMultiplier As Integer: serviceTypeMultiplier = 1
If isServingAbroad Then serviceTypeMultiplier = 2
' service must fall in the budget year supplied
If newServiceStart > lastMonthOfYear Or newServiceEnd < firstMonthOfYear Then
Units = units
Exit Function
End If
If isMarried Then
units = serviceMonthsThisYear
ElseIf eighteenthBirthday < thisYearServiceStart Then
' this person is already eighteen in the supplied year
units = serviceMonthsThisYear * 0.5
ElseIf eighteenthBirthday > thisYearServiceEnd Then
' this person is under eighteen in the supplied year
units = serviceMonthsThisYear * 0.25
Else
' this person turns eighteen during the supplied year
Dim underEighteenMonths As Integer: underEighteenMonths = (Month(eighteenthBirthday) - Month(thisYearServiceStart))
Dim overEighteenMonths As Integer: overEighteenMonths = serviceMonthsThisYear - underEighteenMonths
units = (underEighteenMonths * 0.25) + (overEighteenMonths * 0.5)
End If
' multiply units by servingAbroad multiplier
units = units * serviceTypeMultiplier
Units = units
End Function
Private Function FirstOfTheMonth(dateToConvert As Date) As Date
FirstOfTheMonth = DateSerial(year(dateToConvert), Month(dateToConvert), 1)
End Function
I have a lookup table (named Table1):
I want to run the function above on each of the rows that match a certain criteria in the lookup table via an array formula.
I've tried this:
=SUM(IF(Table1[Payee]="Bill",Units(G1,"1/2/2003",Table1[Start],Table1[End],Table1[Status],Table1[Location])))
This just gives me #VALUE.
I want it to do like the SUM function. If I had another column in the lookup table called value, I could do this:
=SUM(IF(Table1[Payee]="Bill",Table1[value]))
And it would sum the value column just for the rows that match the criteria ("Bill").
How can I get the same to work with my custom function? I would prefer not to have to modify the custom function. The array formula should run the custom function for each row that matches the criteria. Am I misunderstanding how array functions work in Excel?
Edit:
Just in case it makes a difference, I'm using the latest version of Excel from Office 365 on a Mac.
Edit 2:
Added the full code from the function. Basically the function takes the months served in the budgetYear and applies a weight to them. Months under eighteen get 0.25 weight, months eighteen or older get 0.5 weight, and married gets 1 weight. If isServingAbroad is TRUE, the result gets multiplied by 2.