Updated
The EPG is the programme guide: what is on now, and what is on next. It is not a separate subscription. It arrives with the playlist, or from an XMLTV link, depending on the app.
When you do not need a second URL
TiviMate, Smarters, XCIPTV, iPlayTV and GSE usually build the guide from Xtream Codes. Log in, wait for the first sync, and open the guide tab. If it is already full, leave it. Set the device clock to automatic and the timezone to London.
When you do
Paste the XMLTV URL if the guide stays blank. Smart IPTV and the other MAC-based TV apps have an EPG box on their website, next to the playlist. Official Kodi has its own XMLTV field inside PVR IPTV Simple Client. Set that field to a remote path, not a file on the device, then restart Kodi.
VLC does not show this guide. Use it to test a stream, then watch in a player that has an EPG if the listings matter to you.
Times one hour out
An offset of 0 is correct for the UK when the device timezone is London. The usual mistake is a manual clock, or an offset left on +1 from an old setup. After the clocks change, reboot the device so it picks up the new time, then refresh the guide once. Do not refresh it every few minutes. A daily update is enough.
The field names for each device are in the installation guides.