diff --git a/src/portable_agent/models/proposal.py b/src/portable_agent/models/proposal.py
index ea1d386..84b12c8 100644
--- a/src/portable_agent/models/proposal.py
+++ b/src/portable_agent/models/proposal.py
@@ -29,8 +29,8 @@ class CalendarEvent(BaseModel):
     model_config = ConfigDict(extra="forbid")
 
     title: str = Field(min_length=1, max_length=200, pattern=r".*\S.*")
-    start_at: datetime = Field(alias="startAt")
-    end_at: datetime = Field(alias="endAt")
+    start_at: str = Field(alias="startAt")
+    end_at: str = Field(alias="endAt")
     time_zone: str = Field(
         alias="timeZone",
         max_length=100,
@@ -48,9 +48,14 @@ def check_date_type(cls, value: object) -> object:
 
     @model_validator(mode="after")
     def check_time(self) -> Self:
-        if self.start_at.tzinfo is None or self.end_at.tzinfo is None:
+        try:
+            start_at = datetime.fromisoformat(self.start_at)
+            end_at = datetime.fromisoformat(self.end_at)
+        except ValueError as error:
+            raise ValueError("startAt and endAt must be valid date-time values") from error
+        if start_at.tzinfo is None or end_at.tzinfo is None:
             raise ValueError("startAt and endAt must include an offset")
-        if self.end_at <= self.start_at:
+        if end_at <= start_at:
             raise ValueError("endAt must be after startAt")
         if len(self.attendees) != len(set(self.attendees)):
             raise ValueError("attendees must be unique")
diff --git a/tests/test_calendar_event.py b/tests/test_calendar_event.py
new file mode 100644
index 0000000..aa0d387
--- /dev/null
+++ b/tests/test_calendar_event.py
@@ -0,0 +1,17 @@
+from portable_agent.models.proposal import CalendarEvent
+
+
+def test_calendar_event_when_offset_is_set_should_keep_original_text() -> None:
+    start = "2026-09-12T10:00:00+03:00"
+    end = "2026-09-12T10:30:00+03:00"
+
+    event = CalendarEvent(
+        title="Compose check",
+        startAt=start,
+        endAt=end,
+        timeZone="Europe/Moscow",
+    )
+
+    assert isinstance(event.start_at, str)
+    assert event.start_at == start
+    assert event.end_at == end