About time  In the Islamic calendar, every month starts with a sighting of the New Moon. Not just a little symbol appearing in a diary, but someone somewhere actually has to see it. Except that it's impossible to see the New Moon when it's really new, because it's invisible. It normally takes at least 16-18 hours for the thinnest sliver of the crescent moon to appear, and to be spotted. The new month then starts at the following sunset, because all Islamic days begin at sunset.
Example: there was a New Moon yesterday, at 9:18am GMT, but nobody will have seen the crescent moon until very early this morning (about now in fact). This means a new month will start at sunset tonight, and tomorrow will be the 1st day of Murharram. Islamic months always last either 29 or 30 days, but there's no pattern to the sequence and it's not possible to produce accurate calendars looking into the future. Each Islamic year is exactly 12 lunar months long, which is approximately 354 days. This is eleven days adrift of the solar year, which is why months like Ramadan move through the seasons year by year. Islamic years are counted since the Hijra, Mohammed's flight from Mecca in 622 AD.
Example: The next Islamic year will be numbered AH 1425 (anno hegirae, or "in the year of the hijra"). The new year starts on the 1st day of Murharram. Which starts tonight, at sunset. Happy New Year!