Which timezone should I imply in my web service interface?

I'm implementing a REST web service that will be used from all around the world. The serv开发者_如何学Goice will accept "date and time" datatype parameters and serve results also containing "date and time" datatype.

The question is - which timezone do I assume "date and time" belongs to? Do I just force all users to use UTC or is there a better way around?

---

Either include a Timezone property to this service allowing it to specify or assume UTC.