It may be obvious, but perhap worth noting that times can be set using the 24 hour clock, although the phrasing is unnatural. However, if you can get used to it, it skips the "is that morning or afternoon/evening" question, if you forget to specify at the time.
Me: "Alexa, set alarm for half past 18."
Alexa: "Alarm set for 6:30pm."
What you can't do is say "Set alarm for 1800 hours", or "Set alarm for 19:30".
Alexa will always confirm in 12 hour format.