At 20 months, your child is too young for time outs to be effective methods of punishment, although they may be effective in calming him down. If you notice they have a calming effect, you may want to continue, but if they don't, there is no point. Most kids cannot really understand the cause and effect of a time out until they are about 2 years old. Also, do not ignore his hitting. Violence, unlike some other attention-getting behaviors from a child, should never be ignored. I would grab his hands, kneel down so that you are his level and can look him straight in the eye, and tell him with a strong, firm, serious (but not yelling) voice that you know he is angry/frustrated, etc., but he is NOT to hit you, that it hurts, and you will not tolerate it. If he is angry, he needs to tell you he is angry. Do this every time he hits you, without fail.
Then, watch him very carefully. The next time he gets angry with you or Daddy and doesn't hit, but rather uses words (even if he yells and cries), tell him how good he is, how proud you are that he used his words. If he is a visual sort of kid, you might even make some sort of rewards chart and give him a sticker every time he uses his words instead of his hands. After 5 stickers (or something), he gets a small 99 cent toy or something as a reward. Good luck.