What you heard is absolute rubbish. You have a long life. Some wayside astrologer might have told you about it which please do not believe. Read navagrahastro.blogspot.in to know the scope of astrology.
There is nothing of that sort that if you are born on Tuesdays, you are not manglik. Mangal or Mars is a very important planet in the horoscope as it is the planet of courage and initiative to do something in life. It also is the planet of war and aggression. Therefore if Mars is placed in a malefic house it can give bad results. Generally people without much insight into this , create a fear of Mars creating havoc in marital life. Mangal dosha is formed in 5 houses out of 12 houses that you have in a chart. Every house has it own significance. Therefore the mangal dosha of first house is different from mangal dosha forming in the 7th house or 8th house.