Go parser and evaluator for OpenStreetMap opening_hours strings.

A deliberately small, strictly validated subset of the OSM syntax:

• Days: two-letter abbreviations Mo Tu We Th Fr Sa Su (case-insensitive), lists (Mo, We, Fr) and ranges (Mo-Fr); ranges may wrap the week end (Fr-Mo = Fr, Sa, Su, Mo).
• Times: H:MM or HH:MM, hour ≤ 24, minutes ≤ 59; 24:00 (and 00:00) as a close time mean end of day; a close equal to or earlier than its open spans midnight (Su 22:00-02:00; We 08:00-08:00 is a full 24 hours, per OSM semantics).
• Rules: a day group followed by one or more open-close intervals (Mo-Fr 08:00-13:00 14:00-18:00); several groups follow each other, separated by whitespace, , or ;. A trailing day group without times means open the whole day. 24/7 (alone or with a rule tail) is an alias for Mo-Su.

Strict where it matters, lenient where it doesn't. Malformed time syntax and structure fail Parse with ErrInvalidSchedule (wrapped with the reason and byte offset) instead of silently producing a plausible-but-wrong schedule — the worst failure mode for an "open now" filter: out-of-range clock values, missing :, an open time without its close, two times without -. Unknown words (PH off, localized labels) are transparent noise, so real-world feeds still parse. Not supported: month/date ranges, week numbers, public holidays, sunrise/sunset.

Validate at ingest with Parse; the package-level Match never returns an error — an unparsable spec simply never matches.

Package-level Match uses a built-in LRU cache, so repeated calls with the same spec string do not re-parse it. Cache hits are lock-free and scale linearly across goroutines; a miss pays a one-time snapshot rebuild (O(cache size)) per new spec. Suitable for filtering large lists.

now := time.Now()

for _, p := range pharmacies {
    if openhours.Match(p.Schedule, now) {
        results = append(results, p)
    }
}

For the fastest path, pre-parse schedules at ingest (see below) and decompose the instant once per request with TimeOf: MatchAt then costs only the interval scan — about an order of magnitude cheaper per point than Match, with no shared state between goroutines.

wt := openhours.TimeOf(time.Now()) // once per request

for _, p := range pharmacies {
    if p.Schedule.MatchAt(wt) { // p.Schedule is a pre-parsed openhours.Schedule
        results = append(results, p)
    }
}

Parse compiles a spec string once. Use it when the set of schedules is fixed (loaded from config or DB at startup) or when you need more than just a boolean.

l, err := openhours.Parse("Mo-Fr 09:00-20:00; Sa 10:00-18:00")
if err != nil {
    // spec is malformed or contains no recognisable schedule entries
}

// Is it open right now?
open := l.Match(now)

// When does the current open/closed period end?
open, boundary := l.Until(now)
if open {
    fmt.Printf("closes at %s\n", boundary.Format("15:04"))
} else {
    fmt.Printf("opens at %s\n", boundary.Format("15:04"))
}

// All open/close boundaries for the current week.
boundaries, open := l.Split(now)

// Human-readable weekly schedule; current interval is marked with '*'.
fmt.Println(l.Format(now))

now := time.Date(2022, time.November, 13, 21, 32, 0, 0, time.Local) // Sunday

for _, v := range []string{
    "Mo-Tu, Fr 08:00-12:00 14:00-17:00 We 08:00-08:00 Th, Sa-Su 00:00-00:00",
    "Mo-Th 08:00-17:00; Fr 08:00-18:00; Sa 08:00-13:00",
} {
    l, _ := openhours.Parse(v)
    fmt.Printf("%s\n%s %v\n\n", v, l.Format(now), l.Match(now))
}

Mo-Tu, Fr 08:00-12:00 14:00-17:00 We 08:00-08:00 Th, Sa-Su 00:00-00:00
Mon, 07 Nov 08:00-12:00 14:00-17:00
Tue, 08 Nov 08:00-12:00 14:00-17:00
Wed, 09 Nov 08:00-08:00
Thu, 10 Nov 00:00-23:59
Fri, 11 Nov 08:00-12:00 14:00-17:00
Sat, 12 Nov 00:00-23:59
Sun, 13 Nov 00:00*23:59 true

Mo-Th 08:00-17:00; Fr 08:00-18:00; Sa 08:00-13:00
Mon, 07 Nov 08:00-17:00
Tue, 08 Nov 08:00-17:00
Wed, 09 Nov 08:00-17:00
Thu, 10 Nov 08:00-17:00
Fri, 11 Nov 08:00-18:00
Sat, 12 Nov 08:00-13:00 false

• https://pkg.go.dev/github.com/chneau/openhours
• https://pkg.go.dev/github.com/yauhen-l/openhours