Plutonium

If you don't want to use the server's console then you need a third party tool such as IW4MAdmin
Timelimit should be a dvar in the mode's CFG or in your server's CFG like scr_war_timelimit

You can either write a GSC script for it or (recommended) use IW4MAdmin.
It's a server manager that does a lot of useful things, allows you to track players (player count, players connected, max players connected etc.), kick, tempban, ban etc. and with all the features it has it has a built-in and easy to use auto messages feature

https://github.com/RaidMax/IW4M-Admin
https://github.com/RaidMax/IW4M-Admin/releases