Date calculation check in a badi

Hi all I want to do a check calculation in a badi - on an internal table of dates. In normal abap I was able to do it easily. but in oo abap, its not allowing me to do the arethmatic calculation on date fields.Below is the code:

method IF_EX_PT_ABS_REQ~GET_WORKFLOW_ATTRIBS.

begda type pa2001-begda,

end of date1.

data: zbegda type table of date1,

zbegdah type table of date1,

line1 like line of zbegdah.

*data zbegda like pa2001-begda occurs 0 with header line.

*data zbegdah like pa2001-begda occurs 0 with header line.

select begda from pa2001 into table zbegda

where pernr = pernr1.

zbegdah[] = zbegda[].

clear zbegda[].

loop at zbegdah into line1.

add 1 to zbegdah. "<------ THIS LINE IS GIVING ME ERROR

*line1-begda = zbegdah + 1.

append line1 to zbegda.

endloop.

loop at zbegda into line1.

if line1-begda eq bdate.

MESSAGE i074(hrtim_abs_req).

endif.

endloop.

endmethod.

Actually, in my EP leave request workflow I want to give an error message, if the user applies for a leave of say 5th april, and he already has an approved leave of 6th of april - which is stored in infotype 21.

There is a badi - SE18 -> Enhancement Spots PT_ABS_REQ. In this, I am creating implimentation and I am writing the code in the method - IF_EX_PT_ABS_REQ~GET_WORKFLOW_ATTRIBS.

I also want to know that if i declare 2 fields - PERNR1 and bdate ( begin date of leave) in attributes, which I did, would they be passed from EP at runtime? if not, what should I do to get them in my method?

Please suggest.

Thanks

Ribhu