Time grid, time slots and durations

The time grid describes a site's time slots: start times, end times, breaks, lunch break and closures. It serves display, availability checks and automatic generation alike.

Standard storage: the time slot, not the written time

By default, a placed lesson is attached to a slot of the grid. Omniscol stores the position in the grid, then recomputes the displayed times from the site's grid.

A useful consequence: if one day the starting bell changes from 08:15 to 08:10, the lessons placed on that slot follow the new grid without having to edit each lesson one by one.

Grid display mode

The display mode determines how the hours column is presented, without changing lesson placement in any way:

• Lesson hours — each row carries the slot's real times (8:15 – 9:10, 9:15 – 10:15…), as defined by the site's grid.
• Periods — each row carries a period number (P0, P1, P2…), a widespread convention in English-speaking schools.
• Calendar — calendar-style display on round hours (8:00, 9:00, 10:00…), independent of the grid: each lesson sits at its real time on a continuous time background.

Base lesson duration

The base lesson duration is the reference duration of a time slot. It can include the gap between lessons if the institution counts it that way.

Examples:

• 55-minute lessons with a 5-minute gap between them: base duration of 60 minutes;
• regular 1.5-hour lessons: base duration of 90 minutes;
• mostly one-hour lessons, with a few 1.5-hour lessons: base duration of 60 minutes and a suitable lesson unit division.

This duration is used for the configured hour volumes and for generation. Real start and end times can vary within the grid: a recess can make a displayed slot last 55 minutes while remaining attached to a reference duration of 60 minutes.

In the interface, this setting is named Time slot duration and is defined on the General tab of the timetable editor (see General settings).

Typical durations, in order of popularity, are: 60, 45, 50, 120, 90, 30, 180.

Sometimes, in higher-education institutions, lessons always last several hours, but they may start at either 8:00 or 9:00, or last 6 or sometimes 5 hours. In that case the greatest common divisor prevails, and it will usually be a base duration of 60 minutes.

Lesson unit division

The lesson unit division indicates how many sub-parts the base duration can be cut into. It is the minimum time step. A few examples:

• 1: only lessons of a full base duration;
• 1/2: a 30-minute step if the base duration is 60 minutes;
• 1/4: a 15-minute step if the base duration is 60 minutes.

Choose the simplest granularity that covers the actual needs. A finer division allows more flexibility, but increases the number of possibilities to test during generation.

In the interface, this setting is adjusted on the General tab through Time slot division.

In most school systems around the world, the time grid has no subdivision. In France, it is usually 1/2 in secondary education (a few lessons last 1.5 hours). In some training organizations, precision can go down to 10 minutes: that is a base duration of 60 minutes with a 1/6 division.

Position and duration: storage in time steps

A lesson is not recorded with "fixed" times but as a number of time steps on the grid. The step is the base duration divided by the lesson unit division: a base duration of 60 minutes with a 1/2 division gives a 30-minute step. A one-hour lesson then occupies 2 steps, a 1.5-hour lesson occupies 3.

Three notions coexist without merging:

• the reference duration (the base duration) is used to count hour volumes and to display the duration of a lesson;
• the displayed real times (start and end) come from the site's grid, slot by slot;
• storage happens in time steps, at the granularity of the lesson unit division.

This is why the same slot can display 8:15 – 9:10 (real times) while counting as 1 hour (reference duration).

Global slot bans

Some slots must remain banned for the whole institution: Wednesday afternoon, a meeting slot, a reserved range. These bans are set directly on the site's grid, by marking the slot as unavailable: no lesson will be placed there, neither in display nor in generation.

This is a time constraint carried by the grid. On Premium accounts, the same mechanism also allows soft constraints (slots to avoid without closing them). See Time constraints (general system).

Lunch break and offsets

The lunch break is described by a range (for example 12:00 – 14:00) and a lunch duration to preserve. When the duration to preserve is shorter than the range, Omniscol allows a lesson on part of the break, as long as the lunch duration remains possible — before or after the inserted lesson. It is very common for the lunch break to last two hours while a one-hour lesson may still be positioned on it — usually electives where the whole class is not present, or transverse courses where synchronizing several classes is complex. The parts of the class that have no lesson are not considered to be in study hall (for the algorithm, it is not a gap).

The lunch break is also the only case where an irregular slot is treated as a set of distinct time steps. With a 30-minute time step, a base lesson duration of 1 hour and a break range from 12:00 to 13:30, the software considers it as 3 × 30 minutes. You can declare either a single 1.5-hour slot, or 1 hour + 30 minutes, or 30 minutes + 1 hour — it does not matter. In that configuration, the afternoon lessons are shifted by half an hour. If you then declare a minimum lunch duration of half an hour, this leaves the possibility of placing a one-hour lesson on the break, letting the students concerned have lunch either before or after the lesson.

You can also indicate that the canteen opens just before the official break: a "gap" on that slot then does not count as lost time for students.

What matters is modeling the intended rule: open slots, closed slots, flexible lunch break, availability or unavailability. The grid must reflect what the institution actually accepts.