Ask HN: Online time exchanging

What do you think about a site where people can exchange time of their days? I can walk your dog for one hour and you can teach me how to format a pc, do you get it? Every hour you make something for someone gives you one credit, you can use it for anything you are interested in... what do you think about a site like this? do you know something similar?

Great! Thanks! Do you know any example of one of these time banks, but online?