diff --git a/server/app/schemas/block.py b/server/app/schemas/block.py
index c853718..8ad67e3 100644
--- a/server/app/schemas/block.py
+++ b/server/app/schemas/block.py
@@ -1,7 +1,7 @@
-from datetime import datetime
+from datetime import datetime, timezone
 from enum import Enum
 
-from pydantic import BaseModel, ConfigDict
+from pydantic import BaseModel, ConfigDict, field_validator
 
 
 class BlockType(str, Enum):
@@ -18,6 +18,15 @@ class BlockBase(BaseModel):
     block_type: BlockType  # Use the Enum here
     transportation_type: str | None = None
 
+    @field_validator("start_time", "end_time")
+    @classmethod
+    def validate_timezone(cls, v: datetime | None) -> datetime | None:
+        if v is None:
+            return None
+        if v.tzinfo is not None:
+            v = v.astimezone(timezone.utc).replace(tzinfo=None)
+        return v
+
 
 class BlockCreate(BlockBase):
     pass
diff --git a/server/tests/schemas/test_block.py b/server/tests/schemas/test_block.py
new file mode 100644
index 0000000..ecfb3cc
--- /dev/null
+++ b/server/tests/schemas/test_block.py
@@ -0,0 +1,59 @@
+from datetime import datetime, timezone
+from app.schemas.block import BlockCreate, BlockType
+
+def test_timezone_naive_input():
+    # Naive string: 2023-01-01T10:00:00
+    data = {
+        "title": "Naive",
+        "start_time": "2023-01-01T10:00:00",
+        "end_time": "2023-01-01T12:00:00",
+        "detail": "detail",
+        "block_type": "event",
+    }
+    block = BlockCreate(**data)
+    assert block.start_time.tzinfo is None
+    assert block.start_time == datetime(2023, 1, 1, 10, 0, 0)
+    assert block.end_time.tzinfo is None
+    assert block.end_time == datetime(2023, 1, 1, 12, 0, 0)
+
+def test_timezone_aware_utc_input():
+    # Aware UTC string: 2023-01-01T10:00:00Z
+    data = {
+        "title": "Aware UTC",
+        "start_time": "2023-01-01T10:00:00Z",
+        "end_time": "2023-01-01T12:00:00Z",
+        "detail": "detail",
+        "block_type": "event",
+    }
+    block = BlockCreate(**data)
+    # Pydantic parses Z as UTC aware. The validator should convert to naive UTC.
+    assert block.start_time.tzinfo is None
+    assert block.start_time == datetime(2023, 1, 1, 10, 0, 0)
+    assert block.end_time.tzinfo is None
+    assert block.end_time == datetime(2023, 1, 1, 12, 0, 0)
+
+def test_timezone_aware_offset_input():
+    # Aware Offset string: 2023-01-01T19:00:00+09:00 (JST) -> 10:00 UTC
+    data = {
+        "title": "Aware Offset",
+        "start_time": "2023-01-01T19:00:00+09:00",
+        "end_time": "2023-01-01T21:00:00+09:00",
+        "detail": "detail",
+        "block_type": "event",
+    }
+    block = BlockCreate(**data)
+    # The validator should convert to UTC and make naive.
+    assert block.start_time.tzinfo is None
+    assert block.start_time == datetime(2023, 1, 1, 10, 0, 0)
+    assert block.end_time.tzinfo is None
+    assert block.end_time == datetime(2023, 1, 1, 12, 0, 0)
+
+def test_optional_end_time():
+    data = {
+        "title": "Optional End Time",
+        "start_time": "2023-01-01T10:00:00Z",
+        "detail": "detail",
+        "block_type": "event",
+    }
+    block = BlockCreate(**data)
+    assert block.end_time is None