In response to your question, we are given an 18 minute window in which Shabbat candles may be lit or other functions may be performed prior to the actual start of Shabbat. therefore, if candle lighting time is designated at 5:33, you may still light them as late as 5:50 and still allow for an error on the clock. You may light Shabbat candles as early AFTER noon + 1 hour and 18 minutes as you like.

If you recite the bracha over the candles, you have officially commenced Shabbat and all work must cease. However, you may light the candles at the earlier time and NOT recite the bracha until the time Shabbat actually begins (5:33 + :17 + 6:00)

To break these requirements down:

1. Shabbat officially begins 18 minutes after the time that is posted for candle lighting. Therefore, you have a "window" of 17 minutes in which to light them.

For the sake of making the point, if the time posted for lighting candles on Friday night is 5 pm, you may light them as late as 5:17 pm and then commence Shabbat. However, if you light candles at the prescribed time (5 pm), then you have officially begun Shabbat and may not do any more work. If you wait until 5:17 to light candles, Shabbat begins at the precise moment that you have lit the candles. You no longer have an 18 minute "window".

2. Whereas you may bring in Shabbat earlier than the prescribed time, you may light candles one hour and 18 minutes after mid day/noon. If you recite the bracha over the candles, you have begun Shabbat and may no longer do any work...It is, therefore, not advisable to do that.

3. At NO time may you light Shabbat candles AFTER the prescribed time (i.e. 5 pm + 17 minutes). In other words, if posted candle lighting is 5pm and you wait out the 17 minutes, that is the last time in which you may light candles. Never, after that time period.

I hope this clarifies matters for you.