My question is the internal date subroutine is calling dynamically, every time for each record, why don't we use date functions(converting julian to geo)? I am thinking performance will increase, please advice
Some of the COBOL facilities become in-line code while others become subroutine calls during the compile process. I don't know which way the date functions are implemented, so I cannot tell you that changing from your called routine to COBOL intrinsic functions will have ANY performance difference. There might be, there might not be. If you don't test, you won't know. And since your date routines are not installed at our sites, we cannot do the testing for you.
Performance can be a very complicated subject, since there are many many many factors that come into play. The reason we are telling you (repeatedly) to test is that we cannot know the factors influencing performance of the two methods at your site, so we categorically cannot tell you arbitrarily which method will perform better.
You can restate your question 10 ... or 20 ... or 100 times and it's not going to change our answer. Either test both and find out for yourself, or implement one or the other without testing. But do not expect us to just give you an answer -- especially since there's a chance either answer is wrong.