My suggestion would be if you just need to display the previous month text in header of a column then follow these steps:
Create one text variable processed by customer exit.
Write the code in i_step = 1.
Data: v_month(2) TYPE n,
zktx TYPE fcktx,
ym(20) TYPE c.
When 'VAR_TEXT'. --------------(Name of text variable)
v_month = sy-datum+4(2) - 1.
SELECT ktx FROM t247 INTO zktx WHERE spras = 'EN' AND mnr = v_month.
ENDSELECT.
CONCATENATE zktx '-' sy-datum(4) INTO ym.
l_s_range-low = ym.
l_s_range-opt = 'EQ'.
l_s_range-sign = 'I'.
APPEND l_s_range TO e_t_range.
This shall give you the required heading.
Regards,
AL