I Built a Telegram Bot That Solves the "What Time Is It For You?" Problem

#telegram #python #serverless #opensource

"Can we do 6 PM your time? Wait, what time zone are you in again?"

I've had this conversation hundreds of times. I work with people across 7 timezones — from Los Angeles to Bishkek. Every meeting started the same way: open World Time Buddy, convert times, paste the result into Telegram, hope nobody gets confused.

But the real problem wasn't the conversion. It was everything that happens after.

The Actual Problem

Meetings happen in messengers. Not everyone knows each other's email. So nobody adds you to Google Calendar. You get a Zoom link in a Telegram chat, and that's it — no reminder, no calendar event, just a message you'll scroll past and forget.

I needed three things:

Automatic timezone conversion — type the time once, everyone sees it in their city
A Google Calendar link — one click to add the meeting, no email required
DM reminders — because group chat messages get buried

So I built a Telegram bot that does all three from a single message.

How It Works

Send the bot one line:

Strategy Call, 15.03.2026, 18:00, https://zoom.us/j/123456, @alex @maria

It responds with a card like this:

📅 Strategy Call
📆 Saturday, 15 March 2026

🕐 17:00 Riga; Tel-Aviv
🕐 17:00 Rome
🕐 18:00 Istanbul
🕐 20:00 Bishkek
🕐 01:00 Beijing (+1 day)
🕐 08:00 Los Angeles

🔗 https://zoom.us/j/123456

📲 Add to Calendar  ❌ Cancel Reminder

Then, 45 minutes before the meeting, every participant gets a personal DM: "Reminder: Strategy Call starts in 45 minutes." And another one 5 minutes before.

The Stack (All Free Tier)

Service	Role	Cost
Vercel	Hosting (serverless)	$0
Upstash QStash	Delayed reminders	$0
Upstash Redis	User allowlist	$0
Telethon	Personal DMs	$0
pyTelegramBotAPI	Bot interface	$0

Total infrastructure cost: $0/month. Everything runs on free tiers.

The entire bot lives in one file — api/index.py. Deploy with git push. No servers, no Docker, no maintenance.

The Hard Parts

Why Telethon (and why it's risky)

Telegram Bot API has a fundamental limitation: bots can't DM users who haven't started a conversation with the bot. So if I invite @alex to a meeting, the bot can't send Alex a reminder — Alex never messaged the bot.

The workaround: Telethon, a userbot library. It sends DMs from a real Telegram account, not a bot. This bypasses the restriction — but Telegram actively fights spam from userbots. Send too many DMs too fast, and your account gets a FloodWaitError or worse — a permanent ban.

My AI assistant kept warning me: "Don't use Telethon, Telegram will block your account." I used it anyway, with rate limiting and careful batching. So far, no ban.

The 64-Byte Bug

Telegram limits callback_data (the hidden payload in inline buttons) to 64 bytes. QStash message IDs are long. So when the bot tried to create a "Cancel Reminder" button with the full QStash ID, Telegram threw BUTTON_DATA_INVALID.

The fix: store a short numeric key (cancel:1, cancel:2) in the button, and map it to the full QStash ID server-side. Simple, but it took a debugging session to figure out — we only saw the real error after adding detailed error messages to the bot's responses.

DST Is a Nightmare

Timezone conversion sounds simple until you hit Daylight Saving Time. Riga and Tel-Aviv are sometimes in the same timezone, sometimes not. The bot uses Python's zoneinfo with IANA timezone database — proper DST-aware conversion, not hardcoded UTC offsets.

When two cities end up at the same time, the bot merges them: 17:00 Riga; Tel-Aviv.

Architecture

User → Telegram → Vercel (webhook)
                       │
                  api/index.py
                  (Flask + Bot)
                       │
        ┌──────────────┼──────────────┐
        │              │              │
   Upstash Redis    QStash      Telegram API
   (allowlist)   (2 reminders)  (card + buttons)
                       │
                  ┌────┴────┐
                  │         │
             -45 min    -5 min
                  │         │
             /reminder  /reminder
                  │         │
              Bot msg    Bot msg
              + DMs      + DMs
              (Telethon) (Telethon)

Security layers:

Webhook verification via X-Telegram-Bot-Api-Secret-Token
Reminder endpoint protected by custom X-Reminder-Secret header
Dynamic allowlist in Redis (only approved users can create meetings)
Admin roles for managing the allowlist
Input sanitization with html.escape()

What I Learned

1. Infrastructure beats features.

The bot has ~10 features (timezones, reminders, DMs, calendar, allowlist, admin roles, cancel buttons). But the real value is the architecture: one file, zero cost, deploy with git push. Features are easy to add when the foundation is solid.

2. Free tiers are surprisingly generous.

After 2 weeks of usage: 37 QStash messages total, 16 Redis commands, 0 bandwidth. The free tier limits (1,000 QStash messages/day, 500K Redis commands/month) are absurd overkill for a personal tool.

3. The Telethon trade-off is real.

Userbot DMs are the killer feature — and the biggest risk. Telegram can ban your account at any time. For a personal tool, the risk is acceptable. For a product with thousands of users, you'd need a different approach.

4. AI debugging is best when it shows, not tells.

The BUTTON_DATA_INVALID bug was invisible — we only saw "⚠️ Reminder failed." The fix wasn't in the logic; it was adding str(e) to the error message so we could see the real problem in Telegram. AI suggested this in one step.