diff --git a/homeassistant/components/caldav/calendar.py b/homeassistant/components/caldav/calendar.py
index f44a59f18eb..1955751d4fd 100644
--- a/homeassistant/components/caldav/calendar.py
+++ b/homeassistant/components/caldav/calendar.py
@@ -13,7 +13,7 @@ from homeassistant.components.calendar import (
     ENTITY_ID_FORMAT,
     PLATFORM_SCHEMA,
     CalendarEventDevice,
-    calculate_offset,
+    extract_offset,
     get_date,
     is_offset_reached,
 )
@@ -147,9 +147,12 @@ class WebDavCalendarEventDevice(CalendarEventDevice):
         if event is None:
             self._event = event
             return
-        event = calculate_offset(event, OFFSET)
+        (summary, offset) = extract_offset(event["summary"], OFFSET)
+        event["summary"] = summary
         self._event = event
-        self._attr_extra_state_attributes = {"offset_reached": is_offset_reached(event)}
+        self._attr_extra_state_attributes = {
+            "offset_reached": is_offset_reached(get_date(event["start"]), offset)
+        }
 
 
 class WebDavCalendarData:
diff --git a/homeassistant/components/calendar/__init__.py b/homeassistant/components/calendar/__init__.py
index 66a3034938e..4449084373b 100644
--- a/homeassistant/components/calendar/__init__.py
+++ b/homeassistant/components/calendar/__init__.py
@@ -101,15 +101,14 @@ def normalize_event(event: dict[str, Any]) -> dict[str, Any]:
     return normalized_event
 
 
-def calculate_offset(event: dict[str, Any], offset: str) -> dict[str, Any]:
-    """Calculate event offset.
+def extract_offset(summary: str, offset_prefix: str) -> tuple[str, datetime.timedelta]:
+    """Extract the offset from the event summary.
 
-    Return the updated event with the offset_time included.
+    Return a tuple with the updated event summary and offset time.
     """
-    summary = event.get("summary", "")
     # check if we have an offset tag in the message
     # time is HH:MM or MM
-    reg = f"{offset}([+-]?[0-9]{{0,2}}(:[0-9]{{0,2}})?)"
+    reg = f"{offset_prefix}([+-]?[0-9]{{0,2}}(:[0-9]{{0,2}})?)"
     search = re.search(reg, summary)
     if search and search.group(1):
         time = search.group(1)
@@ -121,21 +120,16 @@ def calculate_offset(event: dict[str, Any], offset: str) -> dict[str, Any]:
 
         offset_time = time_period_str(time)
         summary = (summary[: search.start()] + summary[search.end() :]).strip()
-        event["summary"] = summary
-    else:
-        offset_time = datetime.timedelta()  # default it
-
-    event["offset_time"] = offset_time
-    return event
+        return (summary, offset_time)
+    return (summary, datetime.timedelta())
 
 
-def is_offset_reached(event: dict[str, Any]) -> bool:
+def is_offset_reached(
+    start: datetime.datetime, offset_time: datetime.timedelta
+) -> bool:
     """Have we reached the offset time specified in the event title."""
-    start = get_date(event["start"])
-    offset_time: datetime.timedelta = event["offset_time"]
-    if start is None or offset_time == datetime.timedelta():
+    if offset_time == datetime.timedelta():
         return False
-
     return start + offset_time <= dt.now(start.tzinfo)
 
 
diff --git a/homeassistant/components/google/calendar.py b/homeassistant/components/google/calendar.py
index 666096fd8c3..d04d913ae67 100644
--- a/homeassistant/components/google/calendar.py
+++ b/homeassistant/components/google/calendar.py
@@ -11,7 +11,8 @@ from httplib2 import ServerNotFoundError
 from homeassistant.components.calendar import (
     ENTITY_ID_FORMAT,
     CalendarEventDevice,
-    calculate_offset,
+    extract_offset,
+    get_date,
     is_offset_reached,
 )
 from homeassistant.config_entries import ConfigEntry
@@ -178,9 +179,12 @@ class GoogleCalendarEventDevice(CalendarEventDevice):
             _LOGGER.error("Unable to connect to Google: %s", err)
             return
 
-        # Pick the first visible event. Make a copy since calculate_offset mutates the event
+        # Pick the first visible event and apply offset calculations.
         valid_items = filter(self._event_filter, items)
         self._event = copy.deepcopy(next(valid_items, None))
         if self._event:
-            calculate_offset(self._event, self._offset)
-            self._offset_reached = is_offset_reached(self._event)
+            (summary, offset) = extract_offset(self._event["summary"], self._offset)
+            self._event["summary"] = summary
+            self._offset_reached = is_offset_reached(
+                get_date(self._event["start"]), offset
+            )