Parameter in SQL query being misinterpreted

Good day! I've run into a quirk executing an SQL statement and am not
sure if it's a fundamental issue with my query or a bug in psycopg3.

In short, I'm inserting/updating a row using both NOW() and 'NOW() plus
a varying value'. The NOW() value is fine, but what ends up in the
database for the second one is just "NOW() + 4 seconds". I finally
realized this is because of how the command is being processed, the
variable becomes "$4" and apparently the dollar sign is lost.

I'm guessing this is an example of why one of the bullet points at
https://www.psycopg.org/psycopg3/docs/basic/params.html says "The
placeholder must not be quoted". :-/ I would appreciate any advice on
how to work around this while still passing in the value safely.

Here's the command from Python. The value we're looking at is for
"nextupdate". "Stats" is a dictionary which contains "NextDBLog" as an
integer whose value is something between 120 and 800.
=-=-=-=-=-=-=-=-=-
dbRWcur.execute('''
INSERT INTO backendstatus
(nodeid, debug, started, lastupdated, nextupdate, version, platform,
python, cpucores, state, uptime, taskstats)
VALUES
(%(nodeid)s, %(debug)s, %(laststarteddb)s, NOW(), NOW() + INTERVAL
'%(NextDBLog)s SECOND', %(version)s, %(platform)s, %(python)s,
%(cpucores)s, %(state)s, %(uptime)s, %(statsstring)s)
ON CONFLICT
(nodeid)
DO UPDATE SET
debug = excluded.debug,
started = excluded.started,
lastupdated = excluded.lastupdated,
nextupdate = excluded.nextupdate,
version = excluded.version,
platform = excluded.platform,
python = excluded.python,
cpucores = excluded.cpucores,
state = excluded.state,
uptime = excluded.uptime,
taskstats = excluded.taskstats;
''', Stats)
=-=-=-=-=-=-=-=-=-

After the command is processed this is the contents of
dbRWcur._query.query.decode('utf-8').
=-=-=-=-=-=-=-=-=-
INSERT INTO backendstatus
(nodeid, debug, started, lastupdated, nextupdate, version, platform,
python, cpucores, state, uptime, taskstats)
VALUES
($1, $2, $3, NOW(), NOW() + INTERVAL '$4 SECOND', $5, $6, $7, $8, $9,
$10, $11)
ON CONFLICT
(nodeid)
DO UPDATE SET
debug = excluded.debug,
started = excluded.started,
lastupdated = excluded.lastupdated,
nextupdate = excluded.nextupdate,
version = excluded.version,
platform = excluded.platform,
python = excluded.python,
cpucores = excluded.cpucores,
state = excluded.state,
uptime = excluded.uptime,
taskstats = excluded.taskstats;
=-=-=-=-=-=-=-=-=-

This is being run on Rocky Linux v9.5 (essentially Red Hat / RHEL). The
relevant installed packages are:
python3-3.9.19-8.el9_5.1.x86_64
python3-psycopg3-3.1.18-4.el9.noarch (from EPEL)
postgresql-server-15.8-2.module+el9.5.0+28955+a22540b0.x86_64

Daniel Johnson
djohnson(at)progman(dot)us