diff --git a/ingest/api/messages.py b/ingest/api/messages.py
index 8dbf497d..ecd1d394 100644
--- a/ingest/api/messages.py
+++ b/ingest/api/messages.py
@@ -26,6 +26,17 @@ def build_messages(message: object, uuid_prefix: str):
     # Set message publication time in RFC3339 format
     # Create UUID for the message, and state message format version
     for json_msg in message:
+
+        # Convert: (start_datetime, end_datetime) => (datetime, period)
+        if "start_datetime" in json_msg["properties"] and "end_datetime" in json_msg["properties"]:
+            json_msg["properties"]["datetime"] = json_msg["properties"]["end_datetime"]
+            start_dt = datetime.fromisoformat(json_msg["properties"]["start_datetime"])
+            end_dt = datetime.fromisoformat(json_msg["properties"]["end_datetime"])
+            period_int = end_dt - start_dt
+            json_msg["properties"]["period"] = "PT" + str(period_int) + "S"
+            json_msg["properties"].pop("start_datetime")
+            json_msg["properties"].pop("end_datetime")
+
         period = json_msg["properties"]["period_int"]
         message_uuid = f"{uuid_prefix}:{str(uuid.uuid4())}"
         json_msg["id"] = message_uuid
diff --git a/ingest/api/model.py b/ingest/api/model.py
index 8a5ebbdf..8104b5cb 100644
--- a/ingest/api/model.py
+++ b/ingest/api/model.py
@@ -242,7 +242,7 @@ class Properties(BaseModel):
         description=("Instrument level above ground in meters."),
     )
     period: str = Field(
-        ...,
+        None,
         description=(
             "Aggregation period for the measurement. Must be provided in ISO8601 duration format."
             "https://www.iso.org/iso-8601-date-and-time-format.html"
@@ -284,7 +284,7 @@ class Properties(BaseModel):
         ),
     )
     datetime: str = Field(
-        ...,
+        None,
         description="Identifies the date/time of the datas being published, in RFC3339 format.",
     )
     start_datetime: Optional[str] = Field(
@@ -345,17 +345,61 @@ def convert_to_cm(self):
         self.level = int(float(self.level) * 100)
         return self
 
+    @model_validator(mode="after")
+    def check_required_datetime_attributes(self) -> "Properties":
+
+        has_start_datetime = self.start_datetime is not None and len(self.start_datetime)
+        has_end_datetime = self.end_datetime is not None and len(self.end_datetime)
+        has_datetime = self.datetime is not None and len(self.datetime)
+        has_period = self.period is not None and len(self.period)
+        if has_datetime:
+            if has_start_datetime or has_end_datetime:
+                raise ValueError("Double required attributes: datetime or start_datetime/end_datetime")
+            if not has_period:
+                raise ValueError("Required attribute: period")
+        else:
+            if not has_start_datetime:
+                raise ValueError("Required attribute: start_datetime")
+            if not has_end_datetime:
+                raise ValueError("Required attribute: end_datetime")
+
+        return self
+
     @model_validator(mode="after")
     def check_datetime_iso(self) -> "Properties":
         try:
-            dt = parser.isoparse(self.datetime)
+            has_start_datetime = self.start_datetime is not None and len(self.start_datetime)
+            has_end_datetime = self.end_datetime is not None and len(self.end_datetime)
+
+            if has_start_datetime and has_end_datetime:
+                st = parser.isoparse(self.start_datetime)
+                et = parser.isoparse(self.end_datetime)
+            else:
+                dt = parser.isoparse(self.datetime)
         except ValueError:
-            raise ValueError(f"{self.datetime} not in ISO format(YYYY-MM-DDTHH:MM:SSZ)")
+            if has_start_datetime and has_end_datetime:
+                raise ValueError(
+                    f"{self.start_datetime} or {self.end_datetime} not in ISO format(YYYY-MM-DDTHH:MM:SSZ)"
+                )
+            else:
+                raise ValueError(f"{self.datetime} not in ISO format(YYYY-MM-DDTHH:MM:SSZ)")
         except Exception as e:
             raise e
 
-        if dt.tzname() != "UTC":
-            raise ValueError(f"Input datetime, {self.datetime}, is not in UTC timezone")
+        if has_start_datetime and has_end_datetime:
+            if et < st:
+                raise ValueError(
+                    f"end_datetime {self.end_datetime} is earlier than start_datetime {self.start_datetime}"
+                )
+            if st.tzname() != "UTC":
+                raise ValueError(f"Input start_datetime, {self.start_datetime}, is not in UTC timezone")
+            if et.tzname() != "UTC":
+                raise ValueError(f"Input end_datetime, {self.end_datetime}, is not in UTC timezone")
+            self.period = "PT" + str((et - st).seconds) + "S"
+        else:
+            if dt.tzname() != "UTC":
+                raise ValueError(f"Input datetime, {self.datetime}, is not in UTC timezone")
+
         return self
 
     @model_validator(mode="after")
diff --git a/ingest/src/ingest/schemas/e-soh-ingest-spec.json b/ingest/src/ingest/schemas/e-soh-ingest-spec.json
index 876a1ae7..adad38ed 100644
--- a/ingest/src/ingest/schemas/e-soh-ingest-spec.json
+++ b/ingest/src/ingest/schemas/e-soh-ingest-spec.json
@@ -268,7 +268,8 @@
                     "allOf": [
                         {
                             "required": [
-                                "datetime"
+                                "datetime",
+                                "period"
                             ]
                         }
                     ]
diff --git a/ingest/src/ingest/schemas/e-soh-message-spec.json b/ingest/src/ingest/schemas/e-soh-message-spec.json
index cf934926..d751f5eb 100644
--- a/ingest/src/ingest/schemas/e-soh-message-spec.json
+++ b/ingest/src/ingest/schemas/e-soh-message-spec.json
@@ -331,7 +331,8 @@
                     "allOf": [
                         {
                             "required": [
-                                "datetime"
+                                "datetime",
+                                "period"
                             ]
                         }
                     ]