Add Timestamp Functions as a postgres til.

postgres/timestamp-functions.md

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+# Timestamp Functions
+
+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:
+
+```postgresql
+select clock_timestamp(), clock_timestamp(), clock_timestamp(), clock_timestamp();
+```