Time

“Must be in the future”, “at least 18 years old” and “within the next 30 days” all depend on what now is. Validity never reads the system clock directly: every rule about the present reads the clock of the configuration. By default it is the system clock in the default time zone.

Trial.kt

data class Trial(
    val birthDate: LocalDate?,
    val startsOn: LocalDate?
) : Validatable {
    override fun validation() = validation {
        ::birthDate required { minAge(18) }
        ::startsOn required {
            futureOrPresent()
            withinNext(30.days)
            constraint("must start this year", code = "thisYear") { it.year == now(LocalDate::now).year }
        }
    }
}

With the clock fixed on 2 November 2026, a trial for someone born in 2010 that starts in January breaks three rules:

val validity = Validity { clock = Clock.fixed(Instant.parse("2026-11-02T12:00:00Z"), ZoneOffset.UTC) }

Trial(birthDate = LocalDate.of(2010, 1, 1), startsOn = LocalDate.of(2027, 1, 10)).validate(validity).violations

Path	Message	Code
`birthDate`	must be at least 18 years old	`minAge`
`startsOn`	must be within the next 30d	`withinNext`
`startsOn`	must start this year	`thisYear`

The rules that read the clock

Rules	On
`past`, `future`, `pastOrPresent`, `futureOrPresent`	dates, times, instants, months, years
`minAge`, `maxAge`	a birth date
`withinNext`, `withinLast`	dates, times and instants
`today`	dates and times
your own, with `now { }`	anything

The rules that compare two values, such as after(other) or weekday(), do not read the clock.

`now`, in your own rules

now(of) reads the current time from the clock of the validation in progress, so a custom rule follows a fixed clock the same way the built-in ones do:

constraint("must start this year", code = "thisYear") { it.year == now(LocalDate::now).year }
constraint("expires too soon") { it.isAfter(now(Instant::now).plusSeconds(3600)) }

of is any function that takes a Clock, such as LocalDate::now, Instant::now or ZonedDateTime::now.

Fixing the clock

In tests
For the application
For one user

Pass a Validity with a fixed clock to the call under test. The default configuration does not change, so tests that run in parallel do not see each other’s clocks:

val validity = Validity { clock = Clock.fixed(Instant.parse("2026-11-02T12:00:00Z"), ZoneOffset.UTC) }

trial.validate(validity)

Set the zone your business runs in, once, at startup:

Validity.configure { clock = Clock.system(ZoneId.of("America/Sao_Paulo")) }

“Today” in Tokyo is not “today” in São Paulo. Validate with the clock of the user’s zone:

booking.validate(Validity { clock = Clock.system(user.zone) })

A zoned or offset date and time is moved to the zone of the clock before today compares its date.

Get started

Writing rules

Beyond the object

Reading the result

Configuration

For AI agents

The rules that read the clock

`now`, in your own rules

Fixing the clock

​The rules that read the clock

​now, in your own rules

​Fixing the clock

The rules that read the clock

`now`, in your own rules

Fixing the clock