1993 Honda Civic ignition timing

ive just had a smog check and it passed, except for the timing ignition. i brought it to the mechanic to have it fixed. they removed everything cause the previous mechanic put the timing belt wrong( not on the mark),so they replaced everything. what causes the timing to go back to 0 from 16 which the smog center recommend.my mechanic told me that they adjusted it 3 times,but still it goes back to 0 after starting the engine. your help is much appriciated.