the first time I saw ansi literals was in June 2004 in a post on otn forum by alex goodmann. I just cannot stop using them, it is so handy!
Whenever I specify a date (or a timestamp or even a time), with Ansi Date, I do not rely on the NLS parameters, nor I do specify a format.
I simply use
I often use Jan 1st, 2000 as an anonymous date.
For avg(txndate), I can use
date '2000-01-01' + avg(txndate-date '2000-01-01')
Other nice literals are timestamps
timestamp '2000-01-01 00:00:00.000000000 Etc/GMT+0' timestamp '2000-01-01 00:00:00'
less usefull, because unsupported as oracle datatype
time '00:00:00.000000000 +00:00' time '00:00:00'
interval '1' day
one more I want to mention
q'[let's quote this]'
all this I found by reading the doc, more than once ! Google+