In the majority of wp.com themes, posts display the publication date (format set in Settings > General). There are several themes, however, in which posts display the publication time as well. Namely:
Chaotic Soul (on single posts, not on main page)
Elegant Grunge (on single posts, not on main page)
Fusion (on single posts, not on main page)
MLB (all three of them)
Pilcrow (on single posts, not on main page)
Sandbox (all four)
Shocking Blue Green
Silver is the New Black
The Journalist v1.9
Under the Influence (on single posts, not on main page)
zBench (on single posts, not on main page)
In many of the themes that normally don’t show the time stamp, you can make it show up by going to Settings > General and selecting the custom date stamp instead of one of the standard ones. Type the options you prefer for day, date, month and year (see Support doc on formatting date and time), and add the code letters G:i to them (thanks to mmadfan for pointing this out).
• Won’t work in Bold Life, Bouquet, Bueno, Chateau, Chunk, Duotone, Esquire, Fauna, iTheme2, Koi, Monochrome, Next Saturday, Oulipo, Pink Touch 2, Quintus, Redoable Lite, Retro MacOS, Unsleepable.
• Will show twice in Black-Letterhead, Kubrick, NotesIL, Sapphire, Sunburn, Vermilion Christmas.