TeX has a primitive register that contains "the number of minutes since midnight"; with this knowledge it's a moderately simple programming job to print the time (one that no self-respecting Plain TeX user would bother with anyone else's code for).
However, LaTeX provides no primitive for "time", so the non-programming LaTeX user needs help.
Two packages are available, both providing ranges of ways of printing the date, as well as of the time: this question will concentrate on the time-printing capabilities, and interested users can investigate the documentation for details about dates.
The datetime package defines two time-printing functions:
\xxivtime (for 24-hour time), \ampmtime (for 12-hour time) and
\oclock (for time-as-words, albeit a slightly eccentric set of
words).
The scrtime package (part of the compendious
KOMA-Script bundle) takes a package option (12h or
24h) to specify how times are to be printed. The command
\thistime then prints the time appropriately (though there's no
am or pm in 12h mode). The \thistime
command also takes an optional argument, the character to separate the
hours and minutes: the default is of course :.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=time