robfig/cron — last day of the month in Go

The cleanest pattern: schedule daily, check if today is the last day, exit early otherwise. The schedule expression stays standard cron; the “last day” logic lives in Go where it's easier to test.

package main

import (
    "time"

    "github.com/robfig/cron/v3"
)

func isLastDayOfMonth(t time.Time) bool {
    return t.AddDate(0, 0, 1).Day() == 1
}

func main() {
    c := cron.New()
    c.AddFunc("0 0 * * *", func() {
        if !isLastDayOfMonth(time.Now()) {
            return
        }
        // ... your last-day-of-month work here
    })
    c.Start()
    select {}
}

If you prefer to keep the “last day” logic in the cron expression itself, register three crons covering the actual last days of each month length:

c := cron.New()
c.AddFunc("0 0 30 4,6,9,11 *", monthEndJob)        // 30-day months
c.AddFunc("0 0 31 1,3,5,7,8,10,12 *", monthEndJob) // 31-day months
c.AddFunc("0 0 28 2 *", monthEndJob)               // February (28)
c.AddFunc("0 0 29 2 *", func() {                   // February (29) — leap years only
    if time.Now().Month() == time.February && isLastDayOfMonth(time.Now()) {
        monthEndJob()
    }
})
c.Start()

The Feb 29 entry needs a runtime check because the literal value 29 matches every Feb 29, not just leap years. The function double-checks “is today truly the last day of February?”.

• github.com/adhocore/gronx — supports L, W, # modifiers and step ranges. Quartz-compatible.
• github.com/gorhill/cronexpr — supports L, W, L-N arithmetic. Last updated less frequently but stable.
• github.com/go-co-op/gocron — uses its own interval API rather than cron strings; supports “last day of month” via scheduler.Cron(...) with a custom format.

If you're already on robfig/cron and just need last-day-of-month support, the runtime-check pattern is probably less disruptive than swapping libraries.

• Timezone — robfig/cron defaults to local time; be explicit with cron.WithLocation(time.UTC) if your job runs in containers.
• Daylight saving — the “last day at midnight” firing can be skipped or doubled when DST transitions happen at month boundaries.
• Idempotency — if you use a runtime check, still make the job idempotent. A retry, a misfire or a container restart could trigger the job twice in the same day.