Email clients a reminder the day before their appointment

import base64
from datetime import datetime, timedelta, timezone
from email.mime.text import MIMEText
from google.oauth2.credentials import Credentials
from googleapiclient.discovery import build
from clickruntime import on_trigger, credentials

DOMAIN = "northwind.example"
RESCHEDULE = "Reply to this email or call 555-0100 to move your appointment."

@on_trigger
def run(event):
    creds = Credentials(credentials.connection("google", "work"))
    cal = build("calendar", "v3", credentials=creds, cache_discovery=False)
    gmail = build("gmail", "v1", credentials=creds, cache_discovery=False)
    start = (datetime.now(timezone.utc) + timedelta(days=1)).replace(hour=0, minute=0, second=0, microsecond=0)
    events = cal.events().list(calendarId="primary", timeMin=start.isoformat(), timeMax=(start + timedelta(days=1)).isoformat(),
                               singleEvents=True).execute().get("items", [])
    sent = 0
    for ev in events:
        when = ev["start"].get("dateTime", ev["start"].get("date"))
        for guest in [a["email"] for a in ev.get("attendees", []) if not a["email"].endswith("@" + DOMAIN)]:
            body = f"A reminder of your appointment tomorrow at {when[11:16]}.\nPlace: {ev.get('location', 'see invite')}\n{RESCHEDULE}"
            mime = MIMEText(body); mime["To"] = guest; mime["Subject"] = f"Reminder: {ev.get('summary', 'your appointment')} tomorrow"
            gmail.users().messages().send(userId="me", body={"raw": base64.urlsafe_b64encode(mime.as_bytes()).decode()}).execute()
            sent += 1
    return {"reminders": sent}