Fix Message.date timezone conversion causing logical errors in time comparison

[Copilot]

The timestamp_to_datetime() function in pyrogram/utils.py was incorrectly converting Unix timestamps to datetime objects, causing Message.date to have wrong timezone information. This led to logical errors when users compared message timestamps with current time.

Problem

The issue occurred when users in non-UTC timezones performed time comparisons:

last_message_time = message.date  # Returns wrong timezone
datetime_now = datetime.now(last_message_time.tzinfo)
inactive_threshold = (datetime_now - last_message_time).total_seconds()
# Result: Messages appeared to be from the future!

For users in Europe/Kiev timezone (UTC+3), a message timestamp that should represent "22:00:12 UTC" was incorrectly converted to "01:00:12 UTC", making messages appear 3 hours newer than they actually were.

Root Cause

The timestamp_to_datetime() function used:

return datetime.fromtimestamp(ts).replace(tzinfo=timezone.utc)

This approach:

1. Creates a naive datetime in the user's local timezone
2. Then incorrectly labels it as UTC with .replace()
3. Results in an offset error equal to the timezone difference

Solution

Changed to the correct implementation:

return datetime.fromtimestamp(ts, tz=timezone.utc)

This properly interprets Unix timestamps as UTC time, ensuring consistent datetime objects regardless of the user's system timezone.

Validation

• ✅ All message dates now consistently show UTC time across different system timezones
• ✅ Time comparison logic works correctly (messages appear from the past, not future)
• ✅ Edge cases handled properly (epoch, large timestamps, None input)
• ✅ No breaking changes to existing API

Fixes #192.

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