Is it wrong to intentionally use the 18 minutes before shkiya before you light Shabbos candles? What if it’s already been 16 or 17 minutes and you lit candles and you’re 99% sure it was still before shkiya, are you still okay or did you do something wrong?
This is a common mistake that people made regarding hilchos shabbos, and people think “oh it is still before shkiya so I can do melacha” that is not true. We have a mitzvah d’orayso to add to the shabbos before and after, therefore we may absolutely not do melacha p until shkiya. in fact there are poskim that write that a woman that waited until closer than two minutes before shkiya should not light her candles, but should have a gentile light them for her instead, because by lighting her candles then she is violating a mitzvah d’orayso of tosefes shabbos (adding to the shabbos)!
Igros Moshe O:CH 1-96, Zachor Vshamor Kabolas Shabbos pg .2, Shvus Yitrzchok 8 pg. 215.