I would just find a website or api already with a time on the page (Not really worth setting it up yourself). Eg: http://www.currenttimestamp.com/ for this website you can ajax request it then load it into xml, and extract time with xpath like : "//textarea—/text()" to extract the timestamp, to get "1412415775" which is a time format, you can google how to measure the time differences between them
I can't figure any of this out, first I can't figure out how to pull the timestamp off any webpage, ie, the one you posted, so I made my own php file that write the current time in unix timestamp on my own webhost, but the only way I could get the AJAX request to pull the time stamp is if I exported my project then uploaded it to my hosting, seems you can only do AJAX requests for files hosted on the same server, how is this going to work if the target is mobile?