Voucher Reference addition of three fields

I want to create a voucher reference which is a string value and it is addition of three string fields

1) Constant String Value : = "11256-02-CDT"
2) This should be the value of the month of voucher date
if i am creating a voucher on 03/03/2013 then this field should be "0313"
Problem:- i am using MonthOfDate system function but this is returning only 3 and not two digits for month andd for year it is returning 2013 and not "13"
3) this should be a manual value

What i did so far is created a sub screen in sales invoice screen and added three field

I am having two problems so far
1) i am using MonthOfDate system function but this is returning only 3 and not two digits
and yearof date function is returning 2013 and not "13"
2) Addition of these three fields in not working in VCH Ref screen

Kindly help.. Code is below

[Part:subScreen]

Lineummy12


[Lineummy12]
Field:Medium Prompt, Dummy12, Dummy123, Dummy1234
Local: Field: Medium Prompt: Info: 'Voucher Reference'

[Fieldummy12]
Use:Short Name Field
set as:"11256-02-CDT-"
Skip:Yes
Storage:InvNumber
Width:10

[Fieldummy123]
Use:Short Name Field
Set as: $$String:$$MonthOfDate:$DATE
Skip:Yes
Storage:InvNumber2
Width:10

[Fieldummy1234]
Use:Short Name Field
Storage:InvNumber3
Width:10


[#Field: VCH Ref]

Set as: If $$IsSales:##SVVoucherType Then $InvNumber1+$InvNumber2+$InvNumber3 Else ""

 

See take a function from cheque printing formula &use it u will get the solution