`at time` returns an incremental that is `Before` when `now () <= time` and `After` when `now () >= time + alarm_precision`. When `now ()` is between `time` and `time + alarm_precision`, `at time` might be `Before` or `After`, due to the fundamental imprecision of the timing wheel. One is guaranteed that an `at` never becomes `After` too early, but it may become `After` up to `alarm_precision` late.

`after span` is `at (Time.add (now ()) span)`.

`type t````=`

`|` `Before`
`|` `After`

``
`include sig ... end`
`val sexp_of_t : t ‑> Base.Sexp.t`