Here's the link - http://www.gpo.gov/fdsys/pkg/GPO-RIDDICK-1992/pdf/GPO-RIDDICK-1992-34.pdf
Now, if the April 15th deadline in the original 1974 act was subsequently amended in later legislation to May 15th ... okay, but I'm not going to waste my time researching such a trivial point.
Facts are:
1. The deadline in the original 1974 act is April 15 ... so that is where the date came from, pursuant to your request.
2. Even if the deadline has now been pushed back by one month, so what ... Harry Reid is still on the hook for preventing the passage of any federal budget resolution for 3 years!
So, there's your answer, killthecat ... you can either say thank you for having your question answered or you can pout like a two-year-old. 

The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence