Allow timezone procs to be closures

lib/pure/times.nim

@@ -129,8 +129,8 @@ type
                      ## The ``times`` module only supplies implementations for the systems local time and UTC.
                      ## The members ``zoneInfoFromUtc`` and ``zoneInfoFromTz`` should not be accessed directly
                      ## and are only exported so that ``Timezone`` can be implemented by other modules.
-    zoneInfoFromUtc*: proc (time: Time): ZonedTime {.nimcall, tags: [], raises: [], benign .}
+    zoneInfoFromUtc*: proc (time: Time): ZonedTime {.nimcall, tags: [], raises: [], benign, closure.}
-    zoneInfoFromTz*:  proc (adjTime: Time): ZonedTime {.nimcall, tags: [], raises: [], benign .}
+    zoneInfoFromTz*:  proc (adjTime: Time): ZonedTime {.nimcall, tags: [], raises: [], benign, closure.}
     name*: string ## The name of the timezone, f.ex 'Europe/Stockholm' or 'Etc/UTC'. Used for checking equality.
                   ## Se also: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
   ZonedTime* = object ## Represents a zooned instant in time that is not associated with any calendar.

tests/stdlib/ttimes.nim

@@ -281,6 +281,30 @@ suite "ttimes":
     test "parseTest":
       runTimezoneTests()
 
+  test "dynamic timezone":
+    proc staticOffset(offset: int): Timezone =
+      proc zoneInfoFromTz(adjTime: Time): ZonedTime =
+          result.isDst = false
+          result.utcOffset = offset
+          result.adjTime = adjTime
+
+      proc zoneInfoFromUtc(time: Time): ZonedTime =
+          result.isDst = false
+          result.utcOffset = offset
+          result.adjTime = fromUnix(time.toUnix - offset)
+
+      result.name = ""
+      result.zoneInfoFromTz = zoneInfoFromTz
+      result.zoneInfoFromUtc = zoneInfoFromUtc
+
+    let tz = staticOffset(-9000)
+    let dt = initDateTime(1, mJan, 2000, 12, 00, 00, tz)
+    check dt.utcOffset == -9000
+    check dt.isDst == false
+    check $dt == "2000-01-01T12:00:00+02:30"
+    check $dt.utc == "2000-01-01T09:30:00+00:00"
+    check $dt.utc.inZone(tz) == $dt
+
   test "isLeapYear":
     check isLeapYear(2016)
     check (not isLeapYear(2015))