Improve timestamp handling and event navigation

server/monlet_server/schemas.py

@@ -5,6 +5,8 @@ from uuid import UUID
 
 from pydantic import BaseModel, ConfigDict, Field, field_validator
 
+from .timeutil import require_utc_datetime
+
 ID_PATTERN = r"^[A-Za-z0-9._:-]+$"
 UUIDV7_PATTERN = r"^[0-9a-f]{8}-[0-9a-f]{4}-7[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$"
 ID_RE = re.compile(ID_PATTERN)
@@ -80,6 +82,11 @@ class HeartbeatRequest(BaseModel):
     features: AgentFeatures
     labels: dict[str, str] = Field(default_factory=dict)
 
+    @field_validator("observed_at")
+    @classmethod
+    def _observed_at_must_be_utc(cls, v: datetime) -> datetime:
+        return require_utc_datetime(v)
+
     @field_validator("labels")
     @classmethod
     def _check_labels(cls, v: dict[str, str]) -> dict[str, str]:
@@ -109,6 +116,11 @@ class CheckResultEvent(BaseModel):
     notifications_enabled: bool = True
     incident_key: str | None = Field(default=None, max_length=256)
 
+    @field_validator("observed_at")
+    @classmethod
+    def _observed_at_must_be_utc(cls, v: datetime) -> datetime:
+        return require_utc_datetime(v)
+
     @field_validator("output")
     @classmethod
     def _check_output_bytes(cls, v: str | None) -> str | None: