diff --git a/joj3_config_generator/models/task.py b/joj3_config_generator/models/task.py
index 52966a8..ededc4e 100644
--- a/joj3_config_generator/models/task.py
+++ b/joj3_config_generator/models/task.py
@@ -16,6 +16,7 @@ from pydantic import (
 from joj3_config_generator.models.common import Memory, Time
 from joj3_config_generator.models.const import (
     DEFAULT_CASE_SCORE,
+    DEFAULT_CLOCK_LIMIT_MULTIPLIER,
     DEFAULT_CPU_LIMIT,
     DEFAULT_FILE_LIMIT,
     DEFAULT_MEMORY_LIMIT,
@@ -143,25 +144,30 @@ class StageFiles(BaseModel):
 class Limit(BaseModel):
     mem: int = DEFAULT_MEMORY_LIMIT
     cpu: int = DEFAULT_CPU_LIMIT
+    time: int = 0
     stdout: int = DEFAULT_FILE_LIMIT
     stderr: int = DEFAULT_FILE_LIMIT
     proc: int = DEFAULT_PROC_LIMIT
 
-    model_config = ConfigDict(validate_assignment=True)
-
-    @field_validator("cpu", mode="before")
+    @field_validator("cpu", "time", mode="before")
     @classmethod
-    def ensure_time(cls, v: Any) -> Time:
+    def ensure_time_type(cls, v: Any) -> Time:
         if isinstance(v, str):
             return Time(v)
-        raise ValueError("Must be a string")
+        raise ValueError(f'Must be a string, e.g., "1s" or "100ms", but got {v}')
 
     @field_validator("mem", "stdout", "stderr", mode="before")
     @classmethod
-    def ensure_mem(cls, v: Any) -> Memory:
+    def ensure_mem_type(cls, v: Any) -> Memory:
         if isinstance(v, str):
             return Memory(v)
-        raise ValueError("Must be a string")
+        raise ValueError(f'Must be a string, e.g., "256m" or "1g", but got {v}')
+
+    @model_validator(mode="after")
+    def set_time_if_not_set(self) -> "Limit":
+        if self.time == 0:
+            self.time = DEFAULT_CLOCK_LIMIT_MULTIPLIER * self.cpu
+        return self
 
 
 class Parser(str, Enum):
diff --git a/joj3_config_generator/transformers/task.py b/joj3_config_generator/transformers/task.py
index 6b2e92e..f635ca5 100644
--- a/joj3_config_generator/transformers/task.py
+++ b/joj3_config_generator/transformers/task.py
@@ -5,12 +5,7 @@ from pathlib import Path, PurePosixPath
 from typing import Any, Callable, Dict, List, Optional, Tuple, Union
 
 from joj3_config_generator.models import result, task
-from joj3_config_generator.models.common import Memory, Time
-from joj3_config_generator.models.const import (
-    DEFAULT_CLOCK_LIMIT_MULTIPLIER,
-    DEFAULT_PATH_ENV,
-    JOJ3_CONFIG_ROOT,
-)
+from joj3_config_generator.models.const import DEFAULT_PATH_ENV, JOJ3_CONFIG_ROOT
 from joj3_config_generator.utils.logger import logger
 
 
@@ -102,15 +97,15 @@ def get_executor_with(
             copy_out=copy_out_files,
             copy_in_cached={file: file for file in cached},
             copy_out_cached=file_export,
-            cpu_limit=Time(task_stage.limit.cpu),
-            clock_limit=DEFAULT_CLOCK_LIMIT_MULTIPLIER * Time(task_stage.limit.cpu),
-            memory_limit=Memory(task_stage.limit.mem),
+            cpu_limit=task_stage.limit.cpu,
+            clock_limit=task_stage.limit.time,
+            memory_limit=task_stage.limit.mem,
             proc_limit=task_stage.limit.proc,
             stderr=result.Collector(
-                name="stderr", pipe=True, max=Memory(task_stage.limit.stderr)
+                name="stderr", pipe=True, max=task_stage.limit.stderr
             ),
             stdout=result.Collector(
-                name="stdout", pipe=True, max=Memory(task_stage.limit.stdout)
+                name="stdout", pipe=True, max=task_stage.limit.stdout
             ),
         ),
         cases=[],
@@ -218,7 +213,7 @@ def fix_diff(
             stdin=stdin,
             args=shlex.split(case.command) if case.command else None,
             cpu_limit=case.limit.cpu,
-            clock_limit=DEFAULT_CLOCK_LIMIT_MULTIPLIER * case.limit.cpu,
+            clock_limit=case.limit.time,
             memory_limit=case.limit.mem,
             proc_limit=task_stage.limit.proc,
         )
diff --git a/tests/convert/basic/task.json b/tests/convert/basic/task.json
index a4097a5..864f918 100644
--- a/tests/convert/basic/task.json
+++ b/tests/convert/basic/task.json
@@ -741,7 +741,7 @@
                                 "pipe": true
                             },
                             "cpuLimit": 1000000000,
-                            "clockLimit": 2000000000,
+                            "clockLimit": 100000000000,
                             "memoryLimit": 134217728,
                             "stackLimit": 0,
                             "procLimit": 50,
diff --git a/tests/convert/basic/task.toml b/tests/convert/basic/task.toml
index 382aa97..e54dbef 100644
--- a/tests/convert/basic/task.toml
+++ b/tests/convert/basic/task.toml
@@ -91,6 +91,7 @@ command="./h7/build/ex2-asan -a"
 copy-in-cwd = false
 files.import = [ "h7/build/ex2-asan" ]
 limit.mem = "128m"
+limit.time = "100s"
 
 diff.score = 10
 
@@ -100,6 +101,7 @@ result-detail.stderr = true
 
 # will be removed as long as the name is fixed
 case0.limit.cpu = "0.5s"
+case0.limit.time = "1s"
 case0.limit.mem = "5m"
 case0.diff.ignore-spaces = true
 #case0.limit.stdout = 8