IS the timer.Once  allways called from a float or can it be from a string?

float or int, string wouldn't make sense being as it is always a number. You can cast to or parse to the former necessary from a string though.

i tryed to parse it na di am getting

The property or indexer `Oxide.Plugins.TeleportTown.Configuration.citytime' cannot be used in this context because the set accessor is inaccessible

(16:33:12) | GROSSE BROCHE[10462545/76561198048672399] was killed by landmine (entity)

string stringVal = args[2];
float fval = float.Parse(stringVal);
_config.citytime = fval;​

That error is from how you are trying to set the config variable mentioned, not from parsing a number.

Also avoid using Parse function, it is unrealiable in terms of errors (if only you don't wrap it in try catch), use TryParse instead.