If you prefer the 24-hour clock format instead of the usual 12-hour format, Amazon recently (and quietly) added the ability to switch between the two on the Echo Show and Echo Spot.
Colloquially known as "military time," the 24-hour clock format isn't quite as popular, but it's used in a few different industries---the military (hence the nickname), aviation, and hospitals. If you use this time format and have a display-based Echo, here's how to switch over.
Open the settings by swiping down from the top of the screen, and then tapping the little gear icon. On the Echo Show, there's a "Settings" label underneath the icon.
From there, select "Home & Clock" option (Echo Dot) "Home Screen" option (Echo Show).
Toggle the "24-Hour Time" switch to enable it.
Your clock will now display the 24-hour format.
Everything else (like your wallpaper and theme) remains the same.