Is a value that does not change calculated for each row?
As long as it is run as a single transaction, the value will be re-used on each line, not recalculated.  I'm not 100% sure about Postgres or SQL Lite but I'm fairly confident (98%) they will work the same.  

In the past when I was more confused about this question, I would pre-fetch the date value to a variable and then reuse the variable in the where clause.  Now I realize that was pointless unless I have more than one place in a longer set of statements where I would like to use the value.  

As a test, move the calculated date value to the select clause and execute against a large table that takes more than a few seconds to return.  You will notice that the first line will have the same date/time value as the last record.