check here Convert date from string
or you can use this methods to convert
-(NSString*)getStringFromDate:(NSDate*)pDate withFormat:(NSString*)pDateFormat{
NSDateFormatter *dtFormatter = [[NSDateFormatter alloc] init];
[dtFormatter setDateFormat:pDateFormat];
[dtFormatter setLocale:[NSLocale systemLocale]];
return [dtFormatter stringFromDate:pDate];}
-(NSDate*)getDateFromString:(NSString*)pStrDate withFormat:(NSString*)pDateFormat{
NSDateFormatter *dtFormatter = [[NSDateFormatter alloc] init];
[dtFormatter setDateFormat:pDateFormat];
[dtFormatter setLocale:[NSLocale systemLocale]];
return [dtFormatter dateFromString:pStrDate];}
-(NSString*)dateStringFromString:(NSString *)dateToConver format:(NSString *)fromFormat toFormat:(NSString *)toFormat{
NSDate *date = [Helper getDateFromString:dateToConver withFormat:fromFormat];
return [Helper getStringFromDate:date withFormat:toFormat];}