Select query with case using multiple when conditions to get Months from datetime columns

//Query

select CONVERT(date, Tally_VOUCHER.Date), CASE

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=4 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 1 else 2 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=5 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 3 else 4 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=6 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 5 else 6 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=7 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 7 else 8 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=8 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 9 else 10 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=9 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 11 else 12 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=10 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 13 else 14 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=11 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 15 else 16 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=12 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 17 else 18 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=1 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 19 else 20 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=2 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 21 else 22 end

  WHEN DATEPART(MONTH,CONVERT(date, Tally_VOUCHER.Date))=3 then

  case when DATEPART(DAY,CONVERT(date, Tally_VOUCHER.Date))<=15 then 23 else 24 end

   

END Month_Num

from Tally_VOUCHER

//Result

Dates               Month_Num

2013-10-31               14

2013-12-31               18

2013-11-29               16

2013-12-04               17

2013-04-25               2

2013-07-05               7

2013-07-31               8

2013-05-15               3

2013-08-27               10

2013-09-24               12

2013-11-04               15

2013-06-29               6

2013-05-31               4

2013-06-10               5

2013-09-01               11