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,SY-DATUM system variable is set with System Time zone in IMG. SY-DALTO system variable is set with the User ’ s Time Default Time of IMG or Time zone of User profile.