On expiry dates for well, anything, I always took that if something expired on say, 20 January, that meant the last day you could participate in the thing was 20 January. As in the day, at 11:59 on 20 January, not 11:59 on 19 January.
So yes, knowing that my monthly discount expired today (the 20th) here I sit to place an order. But NO! It is expired! I might not be so grumpy about it except this is the second time I have missed out using a coupon - and this one was a rarer $30! I also missed the half-price sale on keyboard covers on my date confusions.