The system-supplied possibility to maintain variables used in variants centrally in the table TVARVC only makes sense if the regular (e.g. monthly) update of such variables does not have to be done manually, but can be done automatically.
If you don't have a self-developed ABAP program at hand for this purpose, you can use the SAP Query toolbox as an alternative, where you can also realize an automatic (job-controlled) update of the table variables through (manageable) additional field coding for an appropriately defined InfoSet.
In this context, various function modules have proven helpful for calculating the posting period and fiscal year from the respective system date, such as LAST_DAY_OF_MONTHS, CALCULATE_DATE or DATE_TO_PERIOD_CONVERT.
Best regards
Ulrich Fahrnschon