difference between sy-datum and sy-datlo

data syI have read many of previous postings relkated to a sy-subrc = 8 that I am getting when trying to open a dataset. Some say dataset name is too long, I do not have authority,When the sy function is executed, the software generates the appropriate function call in the ABAP for the ABAP data flow (appends SY-to the SAP_variable > that you specify)