Victor:

You don't say what format your Total Hours text column is in but if its in a
format hh:nn such as 7:24 (for 7 hours 24 minutes) you can use the CDate
function to return it as a Date/Time data type provided that that no value
exceeds 23:59.

You can use the following function to show the sum of the date/time values
in the format hh:nn:ss

Public Function TimeSum(dblTotalTime As Double) As String

   Const HOURSINDAY = 24
   Dim lngHours As Long
   Dim strMinutesSeconds As String
   Dim strDaysHours As String
   
   'get number of hours
   lngHours = Int(dblTotalTime) * HOURSINDAY + _
       Format(dblTotalTime, "h")
   
   ' get minutes and seconds
   strMinutesSeconds = Format(dblTotalTime, ":nn:ss")
   
   TimeSum = lngHours & strMinutesSeconds
   
End Function

So a query to Sum of the total hours and total pay columns would be along
these lines:

SELECT FORMAT[Date], "yyyy mm") AS [Work Month],
TIMESUM(SUM(CDATE([Total Hours]))) AS [Total Monthly Time],
SUM([Total Pay]) AS [Total Monthly Pay]
GROUP BY FORMAT[Date], "yyyy mm");

If, however the Total Hours values can exceed 23:59, e.g. if it represents
the total hours for a group of employees per day so could be 123:45 for
instance, then you'd need to parse the text value before converting it to a
date/time value.  This can be done with the following function:

Public Function GetTime(strTime As String) As Date

' accepts time as string in format #hh:nn
' i.e time can exceed 23:59
' returns time as date/time data type

   Const DATEZERO As Date = #12:00:00 AM#

   If Left(strTime, InStr(strTime, ":") - 1) \ 24 = 0 Then
       GetTime = CDate(strTime)
   Else
   GetTime = _
       CDate((DATEZERO + Left(strTime, InStr(strTime, ":") - 1) \ 24) & _
       " " & Left(strTime, InStr(strTime, ":") - 1) Mod 24 & ":" & _
       Mid(strTime, InStr(strTime, ":") + 1))
   End If
   
End Function

So the query would be:

SELECT FORMAT([Date], "yyyy mm") AS [Work Month],
TIMESUM(SUM(GETTIME([Date]))) AS [Total Monthly Hours],
SUM([Total Pay]) AS [Total Monthly Pay]
GROUP BY FORMAT([Date], "yyyy mm");

You could of course use a different format to group by and return the month
in, but the above has the advantage of sorting in the correct chronological
order.  If the data spans more than one year, however, the year must be
included in the formatting to distinguish between the same months in
different years.

BTW I'd recommend not using Date as a column name.  It’s the name of a built
in function, so is best avoided.  Something specific like [Work Date] is
better.

The Total Hours column cannot be Null of course, so if a date has no hours
worked and is represented in the table the Total Hours would be 00:00, which
it would be appropriate to set as the columns DefaultValue property.

Incidentally you might notice that if you use the GetTime function on your
text values without using the TimeSum function to sum the values you'll get
individual dates around the start of the 20th century for times over 23:59 as
the result.  this is because Access uses 30 December 1899 00:00:00 as the
origin for its date/time data type so a 'time value' is in reality a
date/time value counting from that point of origin.  Under the skin it’s a 64
bit floating point number with the integer part representing the days and the
fractional part the times of day.

Ken Sheridan
Stafford, England