The 401(k) Plan in question has a one yr. of service requirement and age 21, with immediate entry upon meeting these requirements. An ee was hired and terminated with 1997 with less than 500 hours. He was rehired in 1998. What date of hire do we look at and can hours be addes together to get 1000 ?

You have to read the document. It will have two pertinent sections. One will define what the eligibility computation period is for initial and subsequent years. The other section will describe the procedure for re-hired employees. Read, read, read, there will be your answer!