My 21 month old has temper tantrum 18 hours a day! What can I do?

DMDD. Sounds like dysregulated mood disorder, not terrible 2's because it is so pervasive. Unpublished but submitted research indicates this may be a disorder of brain Glutamate. Amantadine is very effective if this is DMDD. Speak to your pediatrician to be sure that there is not another medical cause of the irritability.
Get help. I would find a therapist with extensive experience in young kids and schedule some sessions.The therapist will likely observe you both in a variety of settings to understand your present level of interaction and then help you find specific ways to move forward. A kid with this level of disruptive behavior needs hands on help.
Give him attention. When he is being good, and don't give eye contact or give in when he has a tantrum. Avoid giving "no" commands, but keep things out of sight and out of reach or use distraction, getting him interested in something else. Avoid, "do you want to get dressed?" and say, " it's time to get dressed, do you want the blue or the green?" two choices, not too many. Is he overtired?
I agree with both . answers. (S)he may have a neurdevelopmental disorder (Autism, language delay), a genetic or other medical disorder of prenatal onset, chronic pain, a disrupted home environment, or a combination of factors. Seek developmental evaluation at http://brightstartsc.com & by referral to a developmental/behavioral pediatrician & a child psychologist. Negative or positive attention reinforces tantrums.