hello there! is anyone going for the M.T interview on 20th?

dear all, can anyone advise me on this? i received a call from PBB this morning saying they are offering me this credit admin in their hq. They want me to go and collect the medical checkup letter 2moro morning i said no problem. THen they ask can i start working as soon as next monday, i said cant cause when i applied for the job, i actually stated that 18th feb onwards then i can only start working. Then she said okay she'll put that down. But after hang up the call, i was thinking, will they change their mind afterwards if i cant start working as per they requested? Or it is quite common?