A bankable deadline system is one where the following rules apply:
- All game Days are guaranteed to last a certain amount of time.
- The first game Day is particularly long.
- Any time left over between the lynch and the deadline of any given Day is carried over ("banked") to lengthen the next Day.
- At the beginning of the game, it is stated that all Days will be three days long.
- Day 1 is set to last for 21 days.
- The players lynch someone after 10 days on Day 1.
- Day 2 is then set to last for 14 days - the three guaranteed days, plus the eleven days not used on Day 1.
- The players lynch someone after 13 days on Day 2.
- Day 3 is then set to last for 4 days - the three guaranteed days, plus the one day not used on Day 2.
The advantage of a bankable deadline system is that it allows for predictable deadlines and allows Day 1 to be as long as it needs to be, but still provides an incentive for players to lynch earlier than the absolute last minute.
In practice, players tend to not care much about banking significantly more time for later Days than they are using on average, but later Days do not need as much time anyway, so this does not tend to be a huge flaw.
This system will occasionally lead to weirdnesses like a 37 day 'deadline' for late-game Days; some mods place a cap on how much time can be banked, to avoid this.