Are eeg's very accurate or not really i had one negative one then a day later a positive for a seizure activity but nothing more in 17 years?

Timing: Eeg's are accurate related to what they can capture when done. If there is no seizure activity during the test, it can be normal. If you have a defined epilepsy disorder, mostly it shows. This is complex also because sometimes seizures are generated from areas in the brain not captured by measuring eeg from scalp. Talk to a neurologist about you personal case.