NSDateFormatter给的日期出错

使用NSDateFormatter修改日期格式，结果日期比给定的日期早一天。
代码如下：

NSString *strDate = [self.mArrEventDate objectAtIndex:i];

NSString *str = [[NSString alloc] init];
str = [strDate stringByReplacingOccurrencesOfString:@"/" withString:@"-"];

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
// ...using a date format corresponding to your date
[dateFormatter setDateFormat:@"dd/MM/yyyy"];

// Parse the string representation of the date
NSDate *date = [dateFormatter dateFromString:strDate];

NSDateFormatter *format1 = [[NSDateFormatter alloc]init];

[format1 setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];

[format1 setDateFormat:@"yyyy-MM-dd"];

NSString *sDate   = [format1 stringFromDate:date];

[format1 setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
估计是时区设置不正确引起的。