Without the parameters of the two occurrences of the "Convert Date to Julian" business function it's not easy to give the full detail. The context of the code could well be important too - what is done with the variables before and/or after the code snippet. The version of JDE may have an impact too. For example, if this code snippet contains a bug, it may have been corrected in a later release.
What is the purpose of your question? Why do you want to find out "what this code is about"? Is there a problem caused by this code? What have you done to help yourself in understanding the code snippet?
It is apparent that the code snippet is from a UBE (Report/Batch process). Which UBE is the code snippet from?
It looks like it is setting up three date related variables: PO jdTO_Date, VA rpt_CurrentDay, VA rpt_DateInJulian. The PO jdTO_Date is set to the date the code is executed. It would appear that VA rpt_CurrentDay is set to the day in the month before PO jdTO_Date. It is difficult to tell what VA rpt_DateInJulian is set to because it is not known what the original value is for the variable. A guess would be that VA rpt_DateInJulian set to the first day of the month in which the code was executed.