Calculando o tempo decorrido usando valores de duas pastas de trabalho diferentes

Question

Aqui está o que estou tentando fazer:

• Eu tenho uma planilha com quatro guias diferentes com dados de linha de lama diferentes (com as colunas (Data, Hora, Mudline e Nível de fluido)
• Eu quero criar uma nova planilha na minha pasta de trabalho atual que terá as quatro colunas "Tempo", "Dias decorridos", "Altura de Mudline" e "Nível de fluido".
• Estou com facilidade preenchendo todas as colunas, exceto a coluna "Dias decorridos". Basicamente eu quero que seja

  Elapsed Days = (the date+time calculated in the column to the left of the cell) − the date+time inputted in Cell B5 of my "Guide" sheet.

Aqui está o meu código até agora:

Sub MudlineData()
    sNamer = "Mudline"

    Dim dateArr(8) As Double

    CreateSheetIf (sNamer)
    Sheets(sNamer).Cells.ClearContents
    Dim currentSheet As Range

    Set currentSheet = Sheets(sNamer).Cells(1, 1) 'Where the current data is going to be pasted

    currentSheet.Offset(0, 0) = "Time"
    currentSheet.Offset(0, 1) = "Elapsed Days"
    currentSheet.Offset(0, 2) = "Mudline(m)"
    currentSheet.Offset(0, 3) = "Fluid Level(m)"

    Set currentSheet = Sheets("Mudline").Cells(2, 1)

    ' gets info from Guide sheet
    file_dir = Sheets("Guide").Cells(2, 2).Value
    filey = Sheets("Guide").Cells(2, 3).Value
    fullFile = file_dir & "\" & filey
    StartT = Sheets("Guide").Cells(2, 5).Value

    Dim wb As Workbook
    Application.ScreenUpdating = False
    Set wb = Workbooks.Open(fullFile, True, True)
    lasty = wb.Worksheets(1).Cells(wb.Worksheets(1).Rows.Count, "A").End(xlUp).Row

    With ThisWorkbook.Worksheets(sNamer)
        For r = 3 To lasty
            'Date and Time
            currentSheet.Offset(0, 0) = wb.Worksheets(1).Cells(r, 1) + wb.Worksheets(1).Cells(r, 2)
            'Elapsed Days
            currentSheet.Offset(0, 1) = currentSheet.Offset(0, 0) - StartT
            'Mudline
            currentSheet.Offset(0, 2) = wb.Worksheets(1).Cells(r, 3)
            'Fluid Level
            currentSheet.Offset(0, 3) = wb.Worksheets(1).Cells(r, 4)
            Set currentSheet = currentSheet.Offset(1, 0)
        Next r
    End With

    wb.Close False
    Set wb = Nothing
    Application.ScreenUpdating = True

End Sub