diff --git a/src/transport_performance/gtfs/calendar.py b/src/transport_performance/gtfs/calendar.py
new file mode 100644
index 00000000..6dea83fe
--- /dev/null
+++ b/src/transport_performance/gtfs/calendar.py
@@ -0,0 +1,86 @@
+"""Cleaners & utilities specific to the calendar table."""
+import calendar
+
+import numpy as np
+import pandas as pd
+
+from transport_performance.utils.defence import (
+    _type_defence,
+    _check_column_in_df,
+)
+
+
+def create_calendar_from_dates(calendar_dates: pd.DataFrame) -> pd.DataFrame:
+    """Use the calendar_dates table to populate a calendar table.
+
+    Use this in cases where a gtfs feed has elected to use a calendar-dates
+    table and no calendar. Only adds values for calendar_dates entries tagged
+    as exception_type 1. Exception_type 2 is not treated. Those entries will
+    are left to override calendar values.
+
+    Parameters
+    ----------
+    calendar_dates : pd.DataFrame
+        The calendar_dates table.
+
+    Returns
+    -------
+    pd.DataFrame
+        A calendar table based on values in the calendar_dates table.
+
+    Raises
+    ------
+    TypeError
+        `calendar_dates` is not a pd.DataFrame
+    IndexError
+        `calendar_dates` does not contain any of the following columns -
+        service_id, date, exception_type.
+
+    """
+    # defence checking
+    _type_defence(calendar_dates, "calendar_dates", pd.DataFrame)
+    exp_cols = ["service_id", "date", "exception_type"]
+    for nm in exp_cols:
+        _check_column_in_df(calendar_dates, nm)
+
+    # create an empty calendar df
+    days = [day.lower() for day in calendar.day_name]
+    new_calendar = pd.DataFrame()
+    new_calendar["service_id"] = calendar_dates["service_id"].unique()
+    day_df = pd.DataFrame(
+        {i: np.zeros(len(new_calendar), dtype=int) for i in days},
+        index=list(range(0, len(new_calendar))),
+    )
+    new_calendar = pd.concat([new_calendar, day_df], axis=1)
+    # create the start and end_date columns
+    new_calendar["start_date"] = ""
+    new_calendar["end_date"] = ""
+    new_calendar.set_index(new_calendar["service_id"], inplace=True)
+    # update this empty calendar with values from calendar_dates
+    for i, r in calendar_dates.iterrows():
+        # only update if calendar_dates exception_type is 1 (adding a service)
+        if r["exception_type"] == 1:
+            date_affected = r["date"]
+            day_affected = pd.to_datetime(date_affected).weekday()
+            # update weekday column entry to show the service runs on that day
+            new_calendar.loc[
+                r["service_id"], new_calendar.columns[day_affected + 1]
+            ] = 1
+            # update the start & end date columns
+            s_date = new_calendar.loc[r["service_id"], "start_date"]
+            if s_date == "":
+                new_calendar.loc[r["service_id"], "start_date"] = date_affected
+            else:
+                s_date = min(s_date, date_affected)
+                new_calendar.loc[r["service_id"], "start_date"] = s_date
+            e_date = new_calendar.loc[r["service_id"], "end_date"]
+            if e_date == "":
+                new_calendar.loc[r["service_id"], "end_date"] = date_affected
+            else:
+                e_date = max(e_date, date_affected)
+                new_calendar.loc[r["service_id"], "end_date"] = e_date
+        else:
+            # Type 2 removes a service and will override the calendar
+            pass
+
+    return new_calendar.reset_index(drop=True)