The rule deciding where a date goes when it lands on a non-working day.
A roll convention decides what happens to a date that falls on a day the relevant system is closed. Move it forward, move it back, or leave it alone — and all three are in use.
It is the mechanism behind a great many wrong dates, because it is invisible in the output. Two systems can hold the same statutory date and produce different answers, and neither will report anything.
The corpus records the rule beside the date for exactly this reason. Where it is not recorded, the honest state is that we do not know whether it moves rather than an assumption that it does.
The same shape of obligation in two countries, both falling on a non-working day, with opposite rules recorded against them.
| 🇬🇧 | Sunday 31 January 2027United Kingdom | Shift rule: noneThe deadline stands on the Sunday. |
| 🇩🇪 | Saturday 31 July 2027Germany | Shift rule: next working dayMoves to Monday 2 August. |
Substitution moves a public holiday off a rest day. A roll convention moves an obligation off a closed day. Same idea, different objects and different rules.
Business day convention is the financial-market name for the same mechanism, with its own named variants.
See also substitution · business day convention · deadline · working day
Only 10 of 640 deadlines currently carry an explicit shift rule. For the rest we record that we do not know rather than assuming one. See what we hold.