Automatic SQL Run Date Range Based Off Of System Time

At work I was trying to come up with sql that could be run to determine the correct range of records to processes. We wanted to bill twice a month, and only grab the transactions that were in the previous half of the month (or prior month) I apologize upfront for having a date stored as a string, but I didn't create this, I just have to work with it. This is what I came up with:

-- This will determine the Date Time String YYYYMMDD to use based off of the SQL system date.  
declare @runDate datetime -- Create a variable for the date, so we don't run into an issue starting to processes at 11:59:99, and getting two different months
declare @currentMonthYYYYMM as VarChar(6)
declare @lastMonthYYYYMM as VarChar(6)
Select @runDate = getDate()

If Month(@runDate) = 1
      Begin
            Select @currentMonthYYYYMM = CAST(year(@runDate) as Varchar(4)) + '01'
            Select @lastMonthYYYYMM = CAST(year(@runDate) - 1 as Varchar(4)) + '12'
      End
Else If Month(@runDate) < 10 
      Begin
            Select @currentMonthYYYYMM = CAST(year(@runDate) as Varchar(4)) + '0' + CAST(Month(@runDate) as Varchar(1))
            Select @lastMonthYYYYMM = CAST(year(@runDate) as Varchar(4)) + '0' + CAST(Month(@runDate)- 1 as Varchar(1))
      End
Else If Month(@runDate) = 10 
      Begin
            Select @currentMonthYYYYMM = CAST(year(@runDate) as Varchar(4)) + CAST(Month(@runDate) as Varchar(2))
            Select @lastMonthYYYYMM = CAST(year(@runDate) as Varchar(4)) + '09'
      End
Else 
      Begin
            Select @currentMonthYYYYMM = CAST(year(@runDate) as Varchar(4)) + CAST(Month(@runDate) as Varchar(2))
            Select @lastMonthYYYYMM = CAST(year(@runDate) as Varchar(4)) + CAST(Month(@runDate) - 1  as Varchar(2))
      End



If day(@runDate) < 16 -- Then Run the Previous month, 16-End of month  Since this is all string dates, YYYY0231 is valid :)
      begin
            SELECT *
            FROM transactions
            WHERE DateToBill Between @lastMonthYYYYMM + '16' AND @lastMonthYYYYMM + '31'
      END
Else
      BEGIN
            SELECT *
            FROM transaction
            WHERE DateToBill Between @currentMonthYYYYMM + '01' AND @currentMonthYYYYMM + '16'
      End

Can you spot any errors or think of a more elegant way? Please let me know.