There are a handful of timestamp functions available in postgres. The most common one is probably now(). This is an alias of transaction_timestamp() which the postgres docs describe as:

Current date and time (start of current transaction)

Two other interesting timestamp functions are statement_timestamp() and clock_timestamp(). The postgres docs describe statement_timestamp() as:

Current date and time (start of current statement)

Using statement_timestamp() throughout a transaction will yield different results from statement to statement.

The postgres docs describe clock_timestamp() as:

Current date and time (changes during statement execution)

Using clock_timestamp() may even yield different results depending on where it appears in a given statement.

Try running something like this to see:

> select clock_timestamp(), clock_timestamp(), clock_timestamp(), clock_timestamp();
        clock_timestamp        |        clock_timestamp        |        clock_timestamp        |        clock_timestamp        
------------------------------- ------------------------------- ------------------------------- ------------------------------
 2015-03-20 14:58:49.832592-05 | 2015-03-20 14:58:49.832592-05 | 2015-03-20 14:58:49.832593-05 | 2015-03-20 14:58:49.832593-05

You'll notice that we see a change in the clock time at the microsecond level mid-way through the statement.

sources: postgres docs and Jack C.