The one place this engine turns an SCXML duration designation - <send delay>'s attribute string, or a delayexpr result - into milliseconds. Wraps Predicator.Duration rather than reimplementing CSS2-style duration parsing.

The delegation is deliberate rather than incidental: one duration vocabulary across the platform, with Predicator.Duration.parse/1 (whole-string, no partial consumption, :error on junk) and to_milliseconds/1 as its only implementation, so a duration means the same thing in a <send delay> as in any expression predicator evaluates. Everything below is a consequence of that, not a second look at it.

The unit set is a superset, delegated as-is

The SCXML schema's delay/delayexpr pattern (\d*(\.\d+)?(ms|s|m|h|d)) recognizes five units: ms, s, m (minutes), h, d. Predicator.Duration.parse/1 recognizes eight: y, mo, w, d, h, m, s, ms - the same five plus y (years), mo (months), and w (weeks). Every unit the two sets share means the same thing in both (m is minutes, not months, in both), so predicator's set is a strict superset of the schema's, and this module delegates it whole rather than re-restricting parsing to the schema's five. A document that writes delay="2w" gets a working two-week delay; nothing here rejects it for being outside the schema pattern, since accepting more than the schema requires is never a conformance violation.

The leading-dot concession

The SCXML schema pattern above allows a bare leading dot - ".5s" is a schema-valid delay value - but Predicator.Duration.parse/1 does not: parse(".5s") is :error while parse("1.5s") is {:ok, ...} (verified against deps/predicator/lib/predicator/duration.ex's own doctests). normalize_leading_dot/1 is the one-line concession this schema pattern requires and predicator 8.0 deliberately does not make. The split is by agreement rather than by omission: ".5s" is schema-valid SCXML but not a duration predicator means to accept, so pre-normalizing it stays this engine's job and does not become a widening of predicator's own grammar. So it rewrites a leading . to 0. before parsing, and ".5s" and "0.5s" parse identically. This is the only rewrite this module performs; every other character reaches Predicator.Duration.parse/1 untouched.