How can I mount a clock on tiles?
You should be able to find a suitable self stick hook of some sort that can hold up the clock. With proper pre-cleaning of the smooth tile surface I would expect a good brand of self stick to stay adhered for a long time.                                                                                                                                                                                     

Some folks have suggested energizing the self stick glue on this type of product by painting on a layer of clear nail polish varnish before before sticking it up to the wall. If you do this make sure to let it dry well before using the hook to hold up the clock.

Note that if the hook style is a bit to long to accomodate the mounting feature on the back of the clock it is easy to cut off part of the hook tip.