diff --git a/tests/lapi/lib.lua b/tests/lapi/lib.lua
index 9c95a0f..b0f1a7f 100644
--- a/tests/lapi/lib.lua
+++ b/tests/lapi/lib.lua
@@ -96,12 +96,22 @@ local function random_locale(fdp)
     return fdp:oneof(locales)
 end
 
+local function err_handler(ignored_msgs)
+    return function(error_msg)
+        for _, ignored_msg in ipairs(ignored_msgs) do
+            local x, _ = string.find(error_msg, ignored_msg)
+            if x then break end
+        end
+    end
+end
+
 return {
     approx_equal = approx_equal,
     bitwise_op = bitwise_op,
-    lua_version = lua_version,
+    err_handler = err_handler,
     lua_current_version_ge_than = lua_current_version_ge_than,
     lua_current_version_lt_than = lua_current_version_lt_than,
+    lua_version = lua_version,
     math_pow = math_pow,
     MAX_INT64 = MAX_INT64,
     MIN_INT64 = MIN_INT64,
diff --git a/tests/lapi/os_date_test.lua b/tests/lapi/os_date_test.lua
new file mode 100644
index 0000000..ac63eac
--- /dev/null
+++ b/tests/lapi/os_date_test.lua
@@ -0,0 +1,45 @@
+--[=[[
+SPDX-License-Identifier: ISC
+Copyright (c) 2023-2025, Sergey Bronnikov.
+
+6.9 – Operating System Facilities
+https://www.lua.org/manual/5.3/manual.html#6.9
+
+LuaJIT 2.1 VM out of memory for `os.date`,
+https://github.com/LuaJIT/LuaJIT/issues/463
+
+Checking a format for os.date may read pass the format string,
+https://www.lua.org/bugs.html#5.3.3-2
+
+os.date throws an error when result is the empty string,
+https://www.lua.org/bugs.html#5.1.1-4
+
+Synopsis: os.date([format [, time]])
+]]=]
+
+local luzer = require("luzer")
+local test_lib = require("lib")
+
+local ignored_msgs = {
+    "invalid conversion specifier",
+}
+
+local function TestOneInput(buf)
+    local fdp = luzer.FuzzedDataProvider(buf)
+    os.setlocale(test_lib.random_locale(fdp), "all")
+    local format = fdp:consume_string(test_lib.MAX_STR_LEN)
+    local time = fdp:consume_integer(test_lib.MIN_INT, test_lib.MAX_INT)
+    local err_handler = test_lib.err_handler(ignored_msgs)
+    local ok, res = xpcall(os.date, err_handler, format, time)
+    if not ok then return end
+    assert(type(res) == "string" or
+           type(res) == "table" or
+           -- Undocumented.
+           type(res) == "number" or
+           res == nil)
+end
+
+local args = {
+    artifact_prefix = "os_date_",
+}
+luzer.Fuzz(TestOneInput, nil, args)
diff --git a/tests/lapi/os_difftime_test.lua b/tests/lapi/os_difftime_test.lua
new file mode 100644
index 0000000..08d9187
--- /dev/null
+++ b/tests/lapi/os_difftime_test.lua
@@ -0,0 +1,33 @@
+--[[
+SPDX-License-Identifier: ISC
+Copyright (c) 2023-2025, Sergey Bronnikov.
+
+6.9 – Operating System Facilities
+https://www.lua.org/manual/5.3/manual.html#6.9
+
+Synopsis: os.difftime(t2, t1)
+]]
+
+local luzer = require("luzer")
+local test_lib = require("lib")
+local MIN_INT = test_lib.MIN_INT
+local MAX_INT = test_lib.MAX_INT
+
+local ignored_msgs = {
+    "number has no integer representation",
+}
+
+local function TestOneInput(buf)
+    local fdp = luzer.FuzzedDataProvider(buf)
+    local t1 = fdp:consume_number(MIN_INT, MAX_INT)
+    local t2 = fdp:consume_number(MIN_INT, MAX_INT)
+    local err_handler = test_lib.err_handler(ignored_msgs)
+    local ok, res = xpcall(os.difftime, err_handler, t1, t2)
+    if not ok then return end
+    assert(type(res) == "number")
+end
+
+local args = {
+    artifact_prefix = "os_difftime_",
+}
+luzer.Fuzz(TestOneInput, nil, args)
diff --git a/tests/lapi/os_getenv_test.lua b/tests/lapi/os_getenv_test.lua
new file mode 100644
index 0000000..8ee9191
--- /dev/null
+++ b/tests/lapi/os_getenv_test.lua
@@ -0,0 +1,21 @@
+--[[
+SPDX-License-Identifier: ISC
+Copyright (c) 2023-2025, Sergey Bronnikov.
+
+6.9 – Operating System Facilities
+https://www.lua.org/manual/5.3/manual.html#6.9
+
+Synopsis: os.getenv(varname)
+]]
+
+local luzer = require("luzer")
+
+local function TestOneInput(buf)
+    local v = os.getenv(buf)
+    assert(type(v) == "string" or v == nil)
+end
+
+local args = {
+    artifact_prefix = "os_getenv_",
+}
+luzer.Fuzz(TestOneInput, nil, args)
diff --git a/tests/lapi/os_setlocale_test.lua b/tests/lapi/os_setlocale_test.lua
new file mode 100644
index 0000000..3a8c4cd
--- /dev/null
+++ b/tests/lapi/os_setlocale_test.lua
@@ -0,0 +1,28 @@
+--[[
+SPDX-License-Identifier: ISC
+Copyright (c) 2023-2025, Sergey Bronnikov.
+
+6.9 – Operating System Facilities
+https://www.lua.org/manual/5.3/manual.html#6.9
+
+Synopsis: os.setlocale(locale [, category])
+]]
+
+local luzer = require("luzer")
+local test_lib = require("lib")
+
+local function TestOneInput(buf)
+    local fdp = luzer.FuzzedDataProvider(buf)
+    local locale = fdp:consume_string(test_lib.MAX_STR_LEN)
+    local category = fdp:oneof({
+        "all", "collate", "ctype", "monetary", "numeric", "time",
+    })
+    local locale_string = os.setlocale(locale, category)
+    assert(type(locale_string) == "string" or
+	       locale_string == nil)
+end
+
+local args = {
+    artifact_prefix = "os_setlocale_",
+}
+luzer.Fuzz(TestOneInput, nil, args)
diff --git a/tests/lapi/os_time_test.lua b/tests/lapi/os_time_test.lua
new file mode 100644
index 0000000..cc6e742
--- /dev/null
+++ b/tests/lapi/os_time_test.lua
@@ -0,0 +1,41 @@
+--[[
+SPDX-License-Identifier: ISC
+Copyright (c) 2023-2025, Sergey Bronnikov.
+
+6.9 – Operating System Facilities
+https://www.lua.org/manual/5.3/manual.html#6.9
+
+Synopsis: os.time([table])
+]]
+
+local luzer = require("luzer")
+local test_lib = require("lib")
+local MAX_INT64 = test_lib.MAX_INT64
+local MIN_INT64 = test_lib.MIN_INT64
+
+local ignored_msgs = {
+    "field 'year' is out-of-bound",
+}
+
+local function TestOneInput(buf)
+    local fdp = luzer.FuzzedDataProvider(buf)
+    os.setlocale(test_lib.random_locale(fdp), "all")
+    local time = {
+        day = fdp:consume_number(MIN_INT64, MAX_INT64),
+        hour = fdp:consume_number(MIN_INT64, MAX_INT64),
+        isdst = fdp:consume_boolean(),
+        min = fdp:consume_number(MIN_INT64, MAX_INT64),
+        month = fdp:consume_number(MIN_INT64, MAX_INT64),
+        sec = fdp:consume_number(MIN_INT64, MAX_INT64),
+        year = fdp:consume_number(MIN_INT64, MAX_INT64),
+    }
+    local err_handler = test_lib.err_handler(ignored_msgs)
+    local ok, res = xpcall(os.time, err_handler, time)
+    if not ok then return end
+    assert(type(res) == "number" or type(res) == "table")
+end
+
+local args = {
+    artifact_prefix = "os_time_",
+}
+luzer.Fuzz(TestOneInput, nil, args)