The date range within stack templates is based on the date on which the stack is run.

Past days is the number of days BEFORE the current date that should be taken into account with the current filter.

Future days is the number of days AFTER the current date that should be taken into account with the current filter.

Based on the requirement to browser a 50 day range looking 10 days into the past and 40 days into the future as visualised below, then 10 would be entered into the Past Days box and 40 would be entered into the Future Days box as shown.