reading the 2.1 release notes these seem to be correct syntax as part
of a select

select dateadd(day, -1, CURRENT_TIMESTAMP) from rdb$database;
select dateadd(-1 day to CURRENT_TIMESTAMP) from rdb$database;

but should I be able to have the following within an SP

period_start = dateadd(day, -1, period_end);
period_start = dateadd(-1 day to period_end);