stupid way to convert datetime to second

The stupid way to convert datetime to second. Hope I can find a better way some days later:)
- (NSTimeInterval)convertDateToSecond:(NSString *)sDate
{
NSDate *dateIn, *dateBase;

NSDateFormatter *formatter = [[[NSDateFormatter alloc] init] autorelease];
[formatter setDateFormat:@"HH:mm:ss"];
dateBase = [formatter dateFromString:@"00:00:00"];
dateIn = [formatter dateFromString:sDate];

return [dateIn timeIntervalSince1970] - [dateBase timeIntervalSince1970];
}