Ah yes, time—always slipping, except when it’s frozen in a default argument. What you’re seeing is expected (if annoying) Python and Prefect behavior: - Python evaluates default arguments at function definition/import time, not at call time. So

datetime.today().strftime(...)

is computed once and baked into the default. - In Prefect 3.x, deployments snapshot those defaults at deploy time. So your “today” becomes “the day I deployed” forever after, unless you pass a value when you run. Fixes 1) Use a sentinel default and compute at run time

from datetime import date
from prefect import flow

@flow
def my_flow(run_date: str | None = None):
    run_date = run_date or date.today().isoformat()
    # do work

2) If you want “today” to be the schedule’s day (not the worker’s clock), use the scheduled start time

from datetime import datetime, timezone
from prefect import flow
from prefect.runtime import flow_run

@flow
def my_flow(run_date: str | None = None):
    if not run_date:
        ts = flow_run.scheduled_start_time  # may be None if triggered ad-hoc
        if ts is None:
            ts = datetime.now(timezone.utc)
        run_date = ts.date().isoformat()
    # do work

Notes - Prefer ISO strings for parameters; they’re JSON friendly. - Avoid naive

datetime.today()

. Use UTC where possible. - If you really want a new default, redeploy or pass the param when triggering a run. Related docs - Schedules in Prefect 3.x: Schedules concept - Deployment versioning (why snapshots stick): Deployment versioning blog If you share your flow signature, I can rewrite it with a safe pattern in about the time it takes a worker to yawn.

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