Loop counter not incrementing for large number of loops

I have a data set with a time stamp in Column A and data in column B. There are four seconds to 30 minutes between data samples. I created VBA code to calculate the one hour average for each row.

The data files can be 800,000 rows long. Everything runs smoothly until around 50,000 rows. After that the average starts to overshoot or undershoot by minutes. Sometimes it provides the average of 45 minutes, and others 1 hour and 20 minutes.

Sub One_hr_Avg()
'' One_hr_Avg Macro
' Determines the cel range of an hour from the starting cell and computes the average of the power across that range.
''
Application.Calculation = xlCalculationManual
    'Set wks = ActiveWorkbook.Sheets("M_FPEB_Pwr_CV3_Plateau")

    'cell_rng: the user defined range of cells that the macro will search throughout for blanks.
    'cell: the current working cell.
    'cell_time: temp cell used to find the row of the cell ~1hr from the current cell.
    Dim cell_rng As Range
    Dim cell As Range
    Dim cell_time As Range

    'cell_start: the the starting time for the 1 hr avg.
    'cell_end: the end time for the 1 hr avg.
    Dim cell_start As Date
    Dim cell_end As Date
    Dim cell_start_temp As Date

    Dim hr_counter As Double
    Dim hr_counter_row As Double

    Dim StartTime As Double
    Dim MinutesElapsed As String

    'Remember time when macro starts
    StartTime = Timer

    'Ask the user for the cell range.
    Set cell_rng = Application.InputBox("Provide a range ($letter$number : $letter$number)", "Cell Range to Fill in Blanks", Type:=8)
    MsgBox "The cells selected were "& cell_rng.Address

    'Go through the cells and set equal to the value in the cell above if the current cell is blank.
    For Each cell In cell_rng

        'Get the time stamp of current cell and calculate the time 1hr ahead.
        cell_start = CDate(Cells(cell.Row, "B").Value)

        cell_start_temp = CDate(Cells(cell.Row, "B").Value)
        cell_end = DateAdd("h", 1, cell_start)
        hr_counter = 1

        Do While CDate(cell_start_temp) < CDate(cell_end)

            cell_start_temp = CDate(Cells(cell.Offset(hr_counter, 0).Row, "B").Value)
            hr_counter = hr_counter + 1

            If (hr_counter + cell.Row) > Cells(Rows.Count, "H").End(xlUp).Row Then
                Exit Do
            End If

        Loop

        hr_counter_row = cell.Row + hr_counter

        cell.Value = "=Average("& "H"& cell.Row & ":"& "H"& hr_counter_row & ")"
        cell.Offset(0, 1).Value = CDate(cell_start_temp)

    Next cell

    'Determine how many seconds code took to run
    MinutesElapsed = Format((Timer - StartTime) / 86400, "hh:mm:ss")

    'Notify user in seconds
    MsgBox "This code ran successfully in "& MinutesElapsed & " minutes", vbInformation

    Application.Calculation = xlCalculationAutomatic

End Sub