Looking for function to derive prior quarter based on the year and month value passed from parameter

public string GetPreviousQuarter(string time)
{
  	var parts = time.Split('M');
	int year = 0;
    int month = 0;
	if (int.TryParse(parts[0], out year) && int.TryParse(parts[1], out month))
	{
		// Determine the current quarter
	    int currentQuarter = (month - 1) / 3 + 1;
	
	    // Calculate the previous quarter
	    int previousQuarter = currentQuarter - 1;
	
	    // If the current quarter is Q1, then the previous quarter is Q4 of the previous year
	    if (previousQuarter < 1)
	    {
	        previousQuarter = 4;
	        year -= 1; // Decrement the year as we're moving to the previous year
	    }		
	    	return $"{year}Q{previousQuarter}";
	}
	else
	{
		return  $"{time} is not a valid time";
	}
}

Public Function GetPreviousQuarter(time As String) As String
    Dim parts() As String = time.Split("M"c)
    Dim year As Integer = 0
    Dim month As Integer = 0
    If Integer.TryParse(parts(0), year) AndAlso Integer.TryParse(parts(1), month) Then
        ' Determine the current quarter
        Dim currentQuarter As Integer = (month - 1) \ 3 + 1

        ' Calculate the previous quarter
        Dim previousQuarter As Integer = currentQuarter - 1

        ' If the current quarter is Q1, then the previous quarter is Q4 of the previous year
        If previousQuarter < 1 Then
            previousQuarter = 4
            year -= 1 ' Decrement the year as we're moving to the previous year
        End If
        Return $"{year}Q{previousQuarter}"
    Else
        Return $"{time} is not a valid time"
    End If
End Function