Date and time on Jolt is split between the core runtime and the jolt-lang/time library. Core owns the java.time value types; formatting and zones are the library (see RFC 0008).
Core carries the #inst reader literal, the java.util / java.text date layer (Date, sql.Date/Timestamp, Calendar, TimeZone, SimpleDateFormat), and the base java.time value types that compute from epoch arithmetic alone: Instant, LocalDate/LocalTime/LocalDateTime, Duration, Period, Year/YearMonth/MonthDay, and the Month/DayOfWeek/ChronoUnit/ ChronoField enums. These work with no dependency — they autoload the first time a program touches one, so date-free programs pay nothing.
The jolt-lang/time library owns the single implementation of everything that formats or names a zone: DateTimeFormatter and FormatStyle, ZoneOffset and ZoneId, ZonedDateTime/OffsetDateTime, java.util.Locale, named-zone offset resolution and DST, localized formatting, and it pulls juxt/tick for the idiomatic API on top. There is no second copy of any of these in core.
The base value types, no dependency
(java.time.Instant/now) ; base, in core
(java.time.LocalDate/of 2020 3 5)
(java.time.LocalDateTime/parse "2020-03-05T13:45:30")
(java.time.Duration/between (java.time.Instant/ofEpochMilli 0)
(java.time.Instant/ofEpochMilli 5000))
(java.time.Period/of 1 2 3)
(java.time.Year/of 2024)
A #inst / java.util.Date converts into the base with .toInstant, one value:
(.toInstant #inst "2020-01-01T00:00:00Z") ; => a java.time.Instant
Reach for a formatting or zone class without the library on your dependencies and the error names the fix rather than leaving a bare "Unknown class":
java.time.format.DateTimeFormatter is provided by the jolt-lang/time library, not
core (RFC 0008). Add io.github.jolt-lang/time to your deps.edn.
Formatting, zones, and tick
;; deps.edn
{:deps {io.github.jolt-lang/time {:git/url "https://github.com/jolt-lang/time.git"
:git/sha "99414a5..."}}}
Declaring the dependency is all that's needed. The library supplies DateTimeFormatter, ZoneOffset/ZoneId, ZonedDateTime/OffsetDateTime, Locale, and the zone/localized layer over the base, and its jolt.time install namespace loads automatically the first time one of those classes is referenced — so tick.core (whose cljc.java-time namespaces resolve them at load) works directly, with no jolt.time require of your own.
(require '[tick.core :as t])
(t/now) ; => an Instant
(t/date "2020-01-01") ; => a LocalDate
(t/>> (t/date "2020-01-01") (t/new-period 3 :months)) ; => 2020-04-01
(t/zone "Europe/Berlin")
(.format (java.time.format.DateTimeFormatter/ofPattern "yyyy-MM-dd")
(java.time.LocalDate/now))
(java.time.ZonedDateTime/parse "2020-07-06T10:59:13.417Z")
How it works
java.time values are opaque host objects registered through Jolt's __register-class-* seams, so they construct, compare, hash, print, sort, and answer instance? exactly like the real classes — and protocols extended to a java.time class (as tick does) dispatch on them. There is one implementation of each class. The base value types are portable Clojure under stdlib/jolt/time/ in core, autoloaded on first use; the library's fmt/zones/zoned namespaces require that base and add formatting and the zone layer. Both halves autoload the same way: on a class miss the runtime loads jolt.time.base from core, and — when the class belongs to the formatting/zone half and jolt.time is resolvable on the source roots — the library's install namespace, then retries the lookup. Each is attempted at most once per process, so a program that never touches a date pays nothing and an unresolvable class still reports the dependency to add. Named-zone offsets and DST come from the OS through a small libc primitive in core (localtime/tzset reading /usr/share/zoneinfo), with a built-in US/EU/AU/NZ rule table as the fallback; localized month and day names come from strftime.
Coverage
Base value-type conformance is covered by core's suite; tick's full test suite and the formatting/zone/DST/localized coverage run in the library's suite. java.util.zip-based helpers are the only gap — there is no java.util.zip shim yet.