I would suggest using a separate database; they're not that difficult to create, consume less resources, and are guaranteed to be persistent to server shutdowns. Concept:
Create a database with columns for guild, time, and (if applicable) map name.
Have an NPC (or function) query the database to check if the user's guild is the owner prior to warping. I'd include code to delete expired entries here, as well.
If there is no owner, ask if the player wants to rent; if yes, and all conditions are met (cost, re-check ownership, etc.), insert an entry into the database (guild, current time + 1 month, map).
(optional) Use an OnPCLoadMapEvent trigger to remove unwanted players from the map.
'gettimetick(2)' retrieves the system time in UNIX epoch time (in seconds). Not really applicable for this unless you want to use UNIX time instead of a DATETIME data type.