As @DStanley mentioned, there is no universally agreed upon standard as to what the timestamp actually represents. You'll have to figure that out based on the context.
In most cases it doesn't really matter much because candlesticks are used as a visual representation. Visually, when you look at a candle, the timestamp is irrelevant because what you're much more interested in the range rather than a particular time in the range (beginning, middle, or end).
Having said that, there is one situation where the timestamp is very relevant, and this is when constructing candlesticks for algorithmic trading.
Since the tick data doesn't come in at predictable, regular intervals, some algorithms require the data to be aggregated into candlesticks. In this case the timestamp will almost always mean the end of the candlestick range. This is because the algorithm is waiting until the end of the candlestick range to be able to make any decisions. With the candlestick timestamp representing the end of the range, that time and the time of any algorithm decisions will coincide.