Our experience with setting timers and reminders on Alexa is that she gets the name of the time spot on but often has trouble understanding the time, no matter whether we specify it as "seven o'clock" (defaulting to the next one, whether it's AM or PM), "seven PM" or "nineteen hundred hours". I'm not sure how the voice recognition that it can recognise fairly complicated product names when adding things to the shopping list, but has trouble understanding times when there are only twelve hour-words to recognise (24 if you include 24-hour) plus "AM", "PM", "in the morning" and "in the evening".
Also, Alexa seems to decide every so often to go into her "by the way, your voice isn't familiar to me - what's your name" routine when given a command, which usually gets an unprintable response from whichever of us is in earshot. Surprisingly she hasn't been programmed to come back with a smart-arse response when told to "f**k off". ;-)