Time granularity must match what the business actually schedules
Aliases: minute step · second picker · scheduling slot
What it is
Which notches a time control can stop on—the hour, five minutes, one minute, one second—is granularity. The smallest interval the business actually distinguishes should line up with that notch: rooms in 15 or 30 minutes, trains on the timetable’s minute, lab logs in seconds. Coarser than the business, a legal time sits between notches and cannot be chosen; finer, people scroll through notches that mean nothing. The issue is how long one notch is, not whether a span may cross midnight, and not how a.m. and p.m. are written.
Why it happens
People read “next notch” as “next thing that matters.” 10:00 to 10:07 is not a bookable slot on most calendars; offering a minute wheel offers values that will not be accepted, then snaps them to 10:15 on submit, so the fineness was fake. Conversely, a flight at 14:03 with a 5-minute grain forces 14:00 or 14:05 and fights the timetable. Granularity also sets the length of a wheel or list: one-second notches are 86,400 options and scanning collapses. The match lives in the scheduling rule, not in an abstract “how precise can time be.”
Where it stops holding
Different tasks in one product want different grains: booking at 30 minutes, a reminder at the minute. One global time widget mismatches one of them. Time-zone conversion can turn an on-the-hour instant into a local time with minutes; grain in the destination zone must follow the converted value. If someone types a finer value (10:07) the business forbids, say which notch it will snap to; do not change it in silence. Stopwatches and audio edits that truly need seconds or milliseconds make a coarse grain the error.
Applying it
- Set notches to the smallest schedulable interval: 15/30 minutes for rooms, the timetable for transit, minutes for reminders, seconds for media.
- Make every value the UI can pick a value submit will still accept; do not offer fake precision that will be snapped away.
- When a typed value is finer than a notch, name the notch it will snap to.
- Verify against times that actually occur in the business: can the control land on them exactly? Then try a midpoint the business does not recognize: is it unselectable, or selectable and gone after submit?