datetime: make timezones only affect formatting

This patch makes `datetime` handle specified `tz` or `tzoffset`
properly. It means timezones are now only affect the way datetime
objects are formatted instead of timestamp value they represent.

The patch involves this changes in the behavior:
1. Creating/setting a `timestamp` value with `tz`/`tzoffset` results
   with a day time corresponding to the timestamp value plus the
   timezone offset. E.g if we create a datetime object with a timestamp
   corresponding to 9:12 o'clock in `+0000` timezone and set
   `tzoffset = -60` the resulting timestamp will represent 8:12 o'clock.
2. Setting a new `tz`/`tzoffset` affects the time of day represented by
   the datetime. E.g. if you update a datetime object of 11:12 o'clock
   having `tzoffset = 180` with `tzoffset = 120` the resulting datetime
   object will represent 10:12.

Closes tarantool#10363

NO_DOC=will be in tarantool/doc#4720

changelogs/unreleased/gh-10363-datetime-handle-timezone.md

@@ -0,0 +1,5 @@
+## bugfix/datetime
+
+* `datetime` methods now handle timezones properly. Now timezone changes
+  only affect the way the object is formatted and don't alter the
+  represented timestamp (gh-10363).

src/lua/datetime.lua

@@ -621,6 +621,9 @@ local function datetime_new(obj)
             s = s - 1
             fraction = fraction + 1
         end
+
+        s = s + (offset or 0) * 60
+
         -- if there are separate nsec, usec, or msec provided then
         -- timestamp should be integer
         if count_usec == 0 then
@@ -939,7 +942,20 @@ local function datetime_parse_from(str, obj)
     -- Override timezone, if it was not specified in a parsed
     -- string.
     if date.tz == '' and date.tzoffset == 0 then
-        datetime_set(date, { tzoffset = tzoffset, tz = tzname })
+        local offset = nil
+
+        if tzoffset ~= nil then
+            offset = get_timezone(tzoffset, 'tzoffset')
+            check_range(offset, -720, 840, 'tzoffset')
+        end
+
+        if tzname ~= nil then
+            offset, date.tzindex = parse_tzname(date.epoch, tzname)
+        end
+
+        if offset ~= nil then
+            time_localize(date, offset)
+        end
     end
 
     return date, len
@@ -1152,13 +1168,23 @@ function datetime_set(self, obj)
         if tzname ~= nil then
             offset, self.tzindex = parse_tzname(sec_int, tzname)
         end
-        self.epoch = utc_secs(sec_int, offset)
+        self.epoch = sec_int
         self.nsec = nsec
         self.tzoffset = offset
 
         return self
     end
 
+    -- Only timezone is changed.
+    if not hms and not ymd then
+        if tzname ~= nil then
+            offset, self.tzindex = parse_tzname(self.epoch, tzname)
+        end
+
+        self.tzoffset = offset
+        return self
+    end
+
     -- normalize time to UTC from current timezone
     time_delocalize(self)
 

test/app-luatest/datetime_test.lua

@@ -2098,3 +2098,56 @@ for supported_by, standard_cases in pairs(UNSUPPORTED_DATETIME_FORMATS) do
         end
     end
 end
+
+-- Test providing a timezone doesn't affect the timestamp
+-- value but instead it affects hours/minutes represented by
+-- datetime objects.
+--
+-- The scenario has been broken before gh-10363.
+pg.test_timestamp_with_tz_in_new_preserves_timestamp = function()
+    local now = dt.now()
+    local tzoffset = now.tzoffset
+
+    local now_from_timestamp_with_tzoffset = dt.new({
+        timestamp = now.timestamp,
+        tzoffset = tzoffset,
+    })
+    local now_from_timestamp_different_tzoffset = dt.new({
+        timestamp = now.timestamp,
+        tzoffset = tzoffset + 60,
+    })
+
+    -- Provided timestamp values aren't affected by tzoffsets.
+    t.assert_equals(now.timestamp, now_from_timestamp_with_tzoffset.timestamp)
+    t.assert_equals(now.timestamp,
+                    now_from_timestamp_different_tzoffset.timestamp)
+
+    -- Hours are updated correspondingly to the provided
+    -- tzoffsets.
+    t.assert_equals(now.hour, now_from_timestamp_with_tzoffset.hour)
+    t.assert_equals(now.hour + 1, now_from_timestamp_different_tzoffset.hour)
+end
+
+-- Test setting a timezone also changes the hour represented
+-- by a datetime object and a corresponding timestamp remains
+-- the same.
+--
+-- The scenario has been broken before gh-10363.
+pg.test_tz_updates_not_change_timestamps = function()
+    local now = dt.now()
+    local tzoffset = now.tzoffset
+
+    local now_with_tzoffset_plus_60 = dt.new({timestamp = now.timestamp})
+                                        :set({tzoffset = tzoffset + 60})
+    local now_with_tzoffset_minus_60 = dt.new({timestamp = now.timestamp})
+                                         :set({tzoffset = tzoffset - 60})
+
+    -- Timestamp values aren't affected by tzoffset changes.
+    t.assert_equals(now.timestamp, now_with_tzoffset_plus_60.timestamp)
+    t.assert_equals(now.timestamp, now_with_tzoffset_minus_60.timestamp)
+
+    -- Hours are updated correspondingly to the tzoffset
+    -- changes.
+    t.assert_equals(now.hour + 1, now_with_tzoffset_plus_60.hour)
+    t.assert_equals(now.hour - 1, now_with_tzoffset_minus_60.hour)
+end

test/app-tap/datetime.test.lua

@@ -2819,27 +2819,28 @@ test:test("Time :set{} operations", function(test)
             'hour 6')
     test:is(tostring(ts:set{ min = 12, sec = 23 }), '2020-11-09T06:12:23+0300',
             'min 12, sec 23')
-    test:is(tostring(ts:set{ tzoffset = -8*60 }), '2020-11-09T06:12:23-0800',
+    test:is(tostring(ts:set{ tzoffset = -8*60 }), '2020-11-08T19:12:23-0800',
             'offset -0800' )
-    test:is(tostring(ts:set{ tzoffset = '+0800' }), '2020-11-09T06:12:23+0800',
+    test:is(tostring(ts:set{ tzoffset = '+0800' }), '2020-11-09T11:12:23+0800',
             'offset +0800' )
-    -- timestamp 1630359071.125 is 2021-08-30T21:31:11.125Z
+    -- Timestamp 1630359071.125 is 2021-08-30T21:31:11.125+0000
+    -- or 2021-08-31T05:31:11.125+0800.
     test:is(tostring(ts:set{ timestamp = 1630359071.125 }),
-            '2021-08-30T21:31:11.125+0800', 'timestamp 1630359071.125' )
-    test:is(tostring(ts:set{ msec = 123}), '2021-08-30T21:31:11.123+0800',
+            '2021-08-31T05:31:11.125+0800', 'timestamp 1630359071.125' )
+    test:is(tostring(ts:set{ msec = 123}), '2021-08-31T05:31:11.123+0800',
             'msec = 123')
-    test:is(tostring(ts:set{ usec = 123}), '2021-08-30T21:31:11.000123+0800',
+    test:is(tostring(ts:set{ usec = 123}), '2021-08-31T05:31:11.000123+0800',
             'usec = 123')
-    test:is(tostring(ts:set{ nsec = 123}), '2021-08-30T21:31:11.000000123+0800',
+    test:is(tostring(ts:set{ nsec = 123}), '2021-08-31T05:31:11.000000123+0800',
             'nsec = 123')
     test:is(tostring(ts:set{timestamp = 1630359071, msec = 123}),
-            '2021-08-30T21:31:11.123+0800', 'timestamp + msec')
+            '2021-08-31T05:31:11.123+0800', 'timestamp + msec')
     test:is(tostring(ts:set{timestamp = 1630359071, usec = 123}),
-            '2021-08-30T21:31:11.000123+0800', 'timestamp + usec')
+            '2021-08-31T05:31:11.000123+0800', 'timestamp + usec')
     test:is(tostring(ts:set{timestamp = 1630359071, nsec = 123}),
-            '2021-08-30T21:31:11.000000123+0800', 'timestamp + nsec')
+            '2021-08-31T05:31:11.000000123+0800', 'timestamp + nsec')
     test:is(tostring(ts:set{timestamp = -0.1}),
-            '1969-12-31T23:59:59.900+0800', 'negative timestamp')
+            '1970-01-01T07:59:59.900+0800', 'negative timestamp')
 end)
 
 test:test("Check :set{} and .new{} equal for all attributes", function(test)