fix(msgpack): strptime() fails east of UTC+12 #41743

Problem:
msgpack#strptime() finds a timestamp by bisecting strftime() output, and
brackets the search with the extreme UTC offsets. The lower bound
subtracts 12 hours, but -12:00 is the westernmost offset, which produces
the *largest* timestamp for a given local time. The easternmost offset
is +14:00, so in any zone east of UTC+12 the search can start above the
target and the function throws:

    internal-start-string:Internal error: start > string

With TZ=GMT-14, four of the five timestamps in msgpack_spec.lua fail,
as do three tests in shada_spec.lua, which reaches the same function
through shada#strings_to_sd().

Solution:
Subtract 14 hours instead. The upper bound is already generous enough at
+14, and widening a bisection bracket downwards cannot change the result
in zones that worked before.

The test disables the python3 provider, because msgpack#strptime() only
uses the Vimscript implementation changed here when no provider answers,
and otherwise hands the work to datetime.strptime().

AI-assisted

test/functional/plugin/msgpack_spec.lua

@@ -735,3 +735,25 @@ describe('autoload/msgpack.vim', function()
     end)
   end)
 end)
+
+describe('autoload/msgpack.vim in an eastern timezone', function()
+  setup(function()
+    clear({
+      -- msgpack#strptime() prefers a Python implementation and only falls back to the
+      -- Vimscript one, which is the one under test, when no provider answers.
+      args = { '-u', 'NORC', '--cmd', 'let g:loaded_python3_provider = 0' },
+      -- POSIX TZ signs are inverted: GMT-14 is UTC+14, the easternmost offset.
+      env = { TZ = 'GMT-14' },
+    })
+  end)
+
+  describe('function msgpack#strptime', function()
+    it('works east of UTC+12 #7625', function()
+      eq(0, nvim_eval('has("python3")'))
+      for _, v in ipairs({ 0, 10, 100000, 204, 1000000000 }) do
+        local time = nvim_eval(('strftime("%%Y-%%m-%%dT%%H:%%M:%%S", %d)'):format(v))
+        eq(v, nvim_eval(('msgpack#strptime("%%Y-%%m-%%dT%%H:%%M:%%S", "%s")'):format(time)))
+      end
+    end)
+  end)
+end)