Why the time difference between two cities changes during the year
People often remember a time difference as a fixed number: "London is five hours ahead of New York." That is true for most of the year, but not all of it. The United States and the United Kingdom change their clocks on different dates, so for a short period each spring and autumn the gap is four hours instead.
Many places never change their clocks at all. Saudi Arabia (Asia/Riyadh), Pakistan (Asia/Karachi) and Japan (Asia/Tokyo) keep the same offset all year. That means their difference from London or New York still shifts whenever those cities move their own clocks.
The reliable approach is to compare time zones on a specific date, using IANA identifiers such as Europe/London rather than city names. Lumara Time uses the IANA database through your browser and PHP, so offsets are looked up for the date you choose instead of being typed in by hand.
Before sending an invitation that falls near a clock change, check the date itself. A recurring weekly meeting can land an hour earlier or later for one side of the call after a change.