[off topic] Date and time fields

Greetings,

Sorry for the off topic post, but the pgsql-sql list has almost no traffic
and I have never received an answer there :(

Could someone give me a basic run down on how to use/do date and time
checking and comparing.  For example, I want to know how long someone is
logged into my site, so each time they request a page I update a "time"
field in my table.  How can I check to see if more than, say 5 or 10
minutes has passed?  I've come to learn that is *can't* be as simple as
something like this:

select * from temp where (logintime + '10 minutes') < time 'now'::datetime;

Also, what is the double colon?  I only use it here because I saw it used
all over in the online docs. ;)

Thank,
Matthew