How to use the "toDate"?

Hi ,

I used "toDate", but without lucky. It displays that "1970-01-01". Why did it display error date ?

And how to get the date from zoho.currenttime, the easiest way.

date Time_to_Date()
{
    aa = (zoho.currenttime.toString("yyyy-MM-dd HH:mm:ss","Asia/Shanghai")).getAlphaNumeric();
    bb = aa.substring(0,8);
    cc = bb.toLong();
    dd = cc.toDate("yyyy-MM-dd");
    return dd;
}