# time.round-to-increment

Rounds a duration to whole billing units. 7 minutes in six-minute units (the
tenth-of-an-hour convention common in legal billing) is 12 rounding up, 6
rounding down and 6 to nearest; 22 minutes in quarter hours is 15 to nearest
and 30 rounding up.

The modes are math.round-div's, which this capability is a thin wrapper over,
so rounding means the same thing here as it does for money:

- `up`: any part of a unit bills as a whole one. The usual rule for
  professional time.
- `down`: only completed units count.
- `half-up`: nearest unit, a tie (exactly half a unit) goes up. This is
  "nearest".
- `half-even`: nearest unit, a tie goes to the even number of units, so ties
  do not bias a long timesheet upwards.

Rounding applies to the one duration you pass. Round each entry and then add,
or add and then round, but know which your terms of business say: seven
3-minute calls rounded up in six-minute units bill 42 minutes one way and 24
the other.

Zero stays zero in every mode. A minimum charge ("at least one unit per call")
is a business rule on top of this, not rounding. Durations are never negative;
minutes below zero and an increment below one are errors.