Actual Start Woks before effective date

Dear All Master Primavera<

I am Prastya,  I am is newbie to used primavera P6 in my project and still to learn and explore for this software,

I have problem during making the schedule, because in my project now already start work but the owner not state the effective date yet

Actually at site already work, but I must to insert this actual date to schedule.

So How to setting this problem ? (like this picture)

Thanks

Regards

Prastya