Mr.Sharma, Z visa is valid for 3 months for a single entry with no specified duration of stay, you are obliged to apply for residence permit at the local public security bureau within a month after entering China. Once you get the Residence Permit before 20 Jan, 2011, you can go back to India and return to China on 18 Feb 2011, the Residence Permit can be used as a multiple entry visa to exit and enter China for many times.