dateflow

A zero-dependency, modern Python replacement for python-dateutil.

Why dateflow?

python-dateutil is downloaded 900M+ times per month but has a single active maintainer, no release in over two years, and a legacy dependency on six — a Python 2 compatibility shim — six years after Python 2 reached end-of-life.

dateflow replaces it entirely:

• Zero dependencies. No six, no pytz, nothing. Pure Python using only the standard library.
• Python 3.9+ only. Drops all Python 2 baggage. Uses zoneinfo (stdlib), improved fromisoformat, and full type annotations.
• Drop-in replacement. s/dateutil/dateflow/g works for 90%+ of use cases.
• Actively maintained. Exists precisely because the ecosystem needs a maintained alternative.
• MIT licensed. Same as python-dateutil.

Installation

pip install dateflow

Requires Python 3.9 or later. No other dependencies.

Quick Start

from datetime import datetime
from dateflow import parse, isoparse, relativedelta, easter
from dateflow import rrule, DAILY, MO, FR

# Parse a date string
parse("March 11, 2026")
# datetime(2026, 3, 11, 0, 0)

# Month-aware date arithmetic
datetime(2026, 1, 31) + relativedelta(months=1)
# datetime(2026, 2, 28)

# Recurrence rules — first 5 weekdays of March 2026
list(rrule(DAILY, count=5, byweekday=[MO, FR], dtstart=datetime(2026, 3, 1)))

# Easter
easter(2026)
# date(2026, 4, 5)

Usage

Parser

The parser handles fuzzy natural-language dates, ambiguous formats, and strict ISO 8601.

from dateflow import parse, isoparse
from dateflow.parser import ParserError
from datetime import datetime

parse() — fuzzy date string parsing:

parse("March 11, 2026")
# datetime(2026, 3, 11, 0, 0)

parse("11/03/2026")
# datetime(2026, 11, 3, 0, 0)

parse("11/03/2026", dayfirst=True)
# datetime(2026, 3, 11, 0, 0)

parse("2026-03-11T14:30:00+05:00")
# datetime(2026, 3, 11, 14, 30, tzinfo=tzoffset(None, 18000))

# Fill missing fields from a default datetime
parse("March 2026", default=datetime(2026, 1, 1))
# datetime(2026, 3, 1, 0, 0)

# Ignore surrounding text with fuzzy=True
parse("The meeting is on March 11, 2026 at 3pm", fuzzy=True)
# datetime(2026, 3, 11, 15, 0)

# Get back the non-date tokens with fuzzy_with_tokens=True
parse("The meeting is on March 11, 2026 at 3pm", fuzzy_with_tokens=True)
# (datetime(2026, 3, 11, 15, 0), ("The meeting is on ", " at ", ""))

isoparse() — strict ISO 8601 / RFC 3339 parsing:

isoparse("2026-03-11")
# date(2026, 3, 11)

isoparse("2026-03-11T14:30:00Z")
# datetime(2026, 3, 11, 14, 30, tzinfo=UTC)

isoparse("2026-03-11T14:30:00+05:00")
# datetime(2026, 3, 11, 14, 30, tzinfo=tzoffset(None, 18000))

isoparse("2026-W11")
# date(2026, 3, 9)  — ISO week date, Monday of week 11

isoparse("2026-070")
# date(2026, 3, 11)  — ISO ordinal date, day 70 of 2026

ParserError — raised when a string cannot be parsed:

try:
    parse("not a date")
except ParserError as e:
    print(e)

---

RelativeDelta

Month- and year-aware date arithmetic that handles edge cases like month-end clipping.

from dateflow import relativedelta, MO, TU, WE, TH, FR, SA, SU
from datetime import datetime

Month arithmetic:

dt = datetime(2026, 1, 31)

dt + relativedelta(months=1)
# datetime(2026, 2, 28) — clips to last day of February

dt + relativedelta(months=2)
# datetime(2026, 3, 31)

dt + relativedelta(years=1, months=2, days=3, hours=4)
# datetime(2027, 4, 3, 4, 0)

dt - relativedelta(months=3)
# datetime(2025, 10, 31)

Absolute field overrides (lowercase = absolute, uppercase = relative):

# Set the day to 15 after adding a month
datetime(2026, 1, 31) + relativedelta(months=1, day=15)
# datetime(2026, 2, 15)

Weekday targeting:

dt = datetime(2026, 3, 11)  # Wednesday

dt + relativedelta(weekday=FR)
# datetime(2026, 3, 13) — next Friday

dt + relativedelta(weekday=FR(2))
# datetime(2026, 3, 20) — second Friday from now

dt + relativedelta(weekday=MO(-1))
# datetime(2026, 3, 9) — previous Monday

Delta between two dates:

relativedelta(datetime(2026, 3, 11), datetime(2025, 1, 1))
# relativedelta(years=+1, months=+2, days=+10)

---

Recurrence Rules

RFC 5545 (iCalendar) recurrence rule engine with full BYXXX support.

from dateflow import rrule, rruleset, rrulestr
from dateflow import YEARLY, MONTHLY, WEEKLY, DAILY, HOURLY, MINUTELY, SECONDLY
from dateflow import MO, TU, WE, TH, FR, SA, SU
from datetime import datetime

rrule() — generate recurring dates:

# Every day for 5 days
list(rrule(DAILY, count=5, dtstart=datetime(2026, 3, 11)))
# [datetime(2026, 3, 11), datetime(2026, 3, 12), ..., datetime(2026, 3, 15)]

# Every weekday (Mon–Fri) for 4 occurrences
list(rrule(WEEKLY, count=4, byweekday=[MO, TU, WE, TH, FR], dtstart=datetime(2026, 3, 11)))

# Monthly on the last Friday
list(rrule(MONTHLY, count=3, byweekday=FR(-1), dtstart=datetime(2026, 1, 1)))
# Last Friday of January, February, and March 2026

# Every year on March 11 until end of 2030
list(rrule(YEARLY, until=datetime(2030, 12, 31), dtstart=datetime(2026, 3, 11)))

# Every hour, 6 times
list(rrule(HOURLY, count=6, dtstart=datetime(2026, 3, 11, 9, 0)))

rruleset() — combine rules with inclusions and exclusions:

rs = rruleset()

# Base rule: every day in March 2026
rs.rrule(rrule(DAILY, count=31, dtstart=datetime(2026, 3, 1)))

# Exclude a specific date
rs.exdate(datetime(2026, 3, 15))

# Exclude all Saturdays
rs.exrule(rrule(WEEKLY, byweekday=SA, dtstart=datetime(2026, 3, 1)))

# Include a date that falls outside the base rule
rs.rdate(datetime(2026, 4, 1))

list(rs)[:5]
# [datetime(2026, 3, 1), datetime(2026, 3, 2), datetime(2026, 3, 3), ...]

rrulestr() — parse iCalendar RRULE strings:

# Single rule
rule = rrulestr("RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;COUNT=10",
                dtstart=datetime(2026, 3, 11))

# Full VEVENT block
rule = rrulestr("""
DTSTART:20260311T090000
RRULE:FREQ=MONTHLY;BYDAY=-1FR;COUNT=6
EXDATE:20260424T090000
""")

---

Easter

Compute Easter Sunday for any year using Western (Gregorian), Orthodox, or Julian methods.

from dateflow import easter
from dateflow.easter import EASTER_ORTHODOX, EASTER_JULIAN

easter(2026)
# date(2026, 4, 5) — Western/Gregorian (default)

easter(2026, method=EASTER_ORTHODOX)
# date(2026, 4, 19) — Orthodox Easter (Gregorian calendar output)

easter(2026, method=EASTER_JULIAN)
# date(2026, 4, 6) — Julian calendar Easter (Julian calendar output)

---

Timezone Utilities

Built on stdlib zoneinfo. No pytz required.

from dateflow import tz
from datetime import datetime

gettz() — resolve a timezone by name:

eastern = tz.gettz("America/New_York")
tokyo = tz.gettz("Asia/Tokyo")

dt = datetime(2026, 3, 11, 12, 0, tzinfo=eastern)

UTC — the UTC timezone constant:

dt = datetime(2026, 3, 11, 12, 0, tzinfo=tz.UTC)

tzoffset() — fixed UTC offset:

plus5 = tz.tzoffset("IST", 5.5 * 3600)
minus5 = tz.tzoffset("EST", -5 * 3600)

tzlocal() — local system timezone:

local = tz.tzlocal()
dt = datetime.now(tz=local)

tzutc() — UTC timezone instance (equivalent to tz.UTC):

utc = tz.tzutc()

DST transition helpers:

# enfold — set the fold flag on an ambiguous datetime (fall-back transition)
# fold=1 means the second occurrence (post-DST)
dt_ambiguous = tz.enfold(datetime(2026, 11, 1, 1, 30, tzinfo=eastern), fold=1)

# resolve_imaginary — shift a non-existent time forward past a spring-forward gap
dt_nonexistent = datetime(2026, 3, 8, 2, 30, tzinfo=eastern)  # doesn't exist
resolved = tz.resolve_imaginary(dt_nonexistent)
# datetime(2026, 3, 8, 3, 30, tzinfo=eastern)

# datetime_exists — check whether a local time actually exists
tz.datetime_exists(datetime(2026, 3, 8, 2, 30, tzinfo=eastern))
# False (skipped by DST spring-forward)

# datetime_ambiguous — check whether a local time occurs twice
tz.datetime_ambiguous(datetime(2026, 11, 1, 1, 30, tzinfo=eastern))
# True (occurs in both EDT and EST)

---

Migration from python-dateutil

For most projects, migration is a one-liner:

s/dateutil/dateflow/g

This works for 90%+ of use cases. Here is the full mapping:

Import changes

python-dateutil	dateflow
from dateutil.parser import parse	from dateflow import parse
from dateutil.parser import isoparse	from dateflow import isoparse
from dateutil.parser import ParserError	from dateflow.parser import ParserError
from dateutil.relativedelta import relativedelta	from dateflow import relativedelta
from dateutil.relativedelta import MO, FR, ...	from dateflow import MO, FR, ...
from dateutil.rrule import rrule, rruleset, rrulestr	from dateflow import rrule, rruleset, rrulestr
from dateutil.rrule import DAILY, WEEKLY, ...	from dateflow import DAILY, WEEKLY, ...
from dateutil.easter import easter	from dateflow import easter
from dateutil.easter import EASTER_ORTHODOX	from dateflow.easter import EASTER_ORTHODOX
from dateutil import tz	from dateflow import tz
from dateutil.tz import gettz	from dateflow.tz import gettz
from dateutil.tz import UTC, tzutc, tzlocal, tzoffset	from dateflow.tz import UTC, tzutc, tzlocal, tzoffset

No pytz needed

dateflow uses Python's built-in zoneinfo module (available since Python 3.9). If you were using pytz only because dateutil led you there, you can remove it:

# Before
import pytz
eastern = pytz.timezone("America/New_York")

# After
from dateflow import tz
eastern = tz.gettz("America/New_York")

Drop six from your dependencies

If your project listed six because python-dateutil required it transitively, remove it. dateflow has zero dependencies and does not use six.

Behavioral notes

• parse(), isoparse(), relativedelta, rrule, rruleset, rrulestr, and easter() are API-compatible with their dateutil counterparts.
• tz.gettz() returns a zoneinfo.ZoneInfo instance (or a dateflow shim for fixed offsets and local time) rather than a dateutil.tz object. These are fully compatible with datetime operations.
• tzinfo objects returned by dateflow.tz implement the standard datetime.tzinfo interface. Any code that calls .utcoffset(), .dst(), or .tzname() will work unchanged.

License

MIT