Synchronised With Internet Time Stamp

All was well until the 16th July. On the 17th July, according to my PC it was the 23rd. For some reason know only to IT technology, my PC decided it didn't like the third week of July and promptly gained a week. I put the date back - only to literally watch it slowly then highlight the date it had decided upon. What happened on the 16th July originally that prompted this - I have no idea - no event log or anything else. Have synchronised with internet time stamp and it tells me it can't because my date is wrong. Every time I change it, it holds for one or two seconds and then literally unhighlights todays date (that I have just changed it to) and highlights the date approximately a week ahead.