replcaement path variable - not able to see variable

HI,

I have query having following date characteristics

1) posting date

2) document date

I want the user to enter posting date on selection screen, and that value should automatically pass to 'document date' also.

Basically passing(copying) the posting date to document date.

I have created a posting date variable (zpstng_date)  as ' interval, manual input, ready for input' and restricted the posting date characteristic with this variable.

Now when I create variable for document date as ' single value, replace with variable, not ready for input,'

I cannot see the posting date variable 'zpstng_date' when I search it in 'replace variable with' tab, so that I can replace the document date with posing date.

Please help

Regards,

Rahul S