Errors out when calling .strftime('%+')

[marc-at-brightnight]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import datetime

data = {'date': [datetime.date(2021, 4, 1),
  datetime.date(2021, 4, 2),
  datetime.date(2021, 4, 3)]}

df = pl.DataFrame(data)

df.with_columns(date_str=pl.col('date').dt.strftime('%+')) # fails

dt_df = df.cast({pl.Date: pl.Datetime})

dt_df.with_columns(date_str=pl.col('date').dt.strftime('%+')) # also fails, though different error

Log output

`df.with_columns(date_str=pl.col('date').dt.strftime('%+'))`

thread '<unnamed>' panicked at crates/polars-core/src/chunked_array/temporal/date.rs:46:50:
called `Result::unwrap()` on an `Err` value: Error
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "/Users/mnhmbp/PycharmProjects/app/venv/lib/python3.10/site-packages/IPython/core/interactiveshell.py", line 3577, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-4-ed1cdc278abd>", line 14, in <module>
    df.with_columns(date_str=pl.col('date').dt.strftime('%+')) # fails
  File "/Users/mnhmbp/PycharmProjects/app/venv/lib/python3.10/site-packages/polars/dataframe/frame.py", line 8310, in with_columns
    return self.lazy().with_columns(*exprs, **named_exprs).collect(_eager=True)
  File "/Users/mnhmbp/PycharmProjects/app/venv/lib/python3.10/site-packages/polars/lazyframe/frame.py", line 1816, in collect
    return wrap_df(ldf.collect(callback))
pyo3_runtime.PanicException: called `Result::unwrap()` on an `Err` value: Error

`dt_df.with_columns(date_str=pl.col('date').dt.strftime('%+'))`

Traceback (most recent call last):
  File "/Users/mnhmbp/PycharmProjects/app/venv/lib/python3.10/site-packages/IPython/core/interactiveshell.py", line 3577, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-6-d81b3df6bd78>", line 1, in <module>
    dt_df.with_columns(date_str=pl.col('date').dt.strftime('%+')) # also fails, though different error
  File "/Users/mnhmbp/PycharmProjects/app/venv/lib/python3.10/site-packages/polars/dataframe/frame.py", line 8310, in with_columns
    return self.lazy().with_columns(*exprs, **named_exprs).collect(_eager=True)
  File "/Users/mnhmbp/PycharmProjects/app/venv/lib/python3.10/site-packages/polars/lazyframe/frame.py", line 1816, in collect
    return wrap_df(ldf.collect(callback))
polars.exceptions.ComputeError: cannot format NaiveDateTime with format '%+'

Issue description

My use case is simply converting a date column to isoformat, similar to called datetime.datetime.utcnow().isoformat(). As such, it would be nice to have a convenience function .isoformat(), that works with both pl.Date and pl.Datetime.

According to the docs, using strftime('%+') seems like the most straightforward way to get this done but I could be missing something.

Expected behavior

df.with_columns(date_str=pl.Series([d.isoformat() for d in df['date'].to_list()]))

┌────────────┬────────────┐
│ date       ┆ date_str   │
│ ---        ┆ ---        │
│ date       ┆ str        │
╞════════════╪════════════╡
│ 2021-04-01 ┆ 2021-04-01 │
│ 2021-04-02 ┆ 2021-04-02 │
│ 2021-04-03 ┆ 2021-04-03 │
└────────────┴────────────┘

dt_df.with_columns(date_str=pl.Series([d.isoformat() for d in dt_df['date'].to_list()]))

┌─────────────────────┬─────────────────────┐
│ date                ┆ date_str            │
│ ---                 ┆ ---                 │
│ datetime[μs]        ┆ str                 │
╞═════════════════════╪═════════════════════╡
│ 2021-04-01 00:00:00 ┆ 2021-04-01T00:00:00 │
│ 2021-04-02 00:00:00 ┆ 2021-04-02T00:00:00 │
│ 2021-04-03 00:00:00 ┆ 2021-04-03T00:00:00 │
└─────────────────────┴─────────────────────┘

Installed versions

--------Version info---------
Polars:               0.20.26
Index type:           UInt32
Platform:             macOS-14.2-arm64-arm-64bit
Python:               3.10.13 (main, Aug 24 2023, 12:59:26) [Clang 15.0.0 (clang-1500.0.40.1)]
----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          3.0.0
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               2024.2.0
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           3.8.3
nest_asyncio:         1.6.0
numpy:                1.23.0
openpyxl:             3.1.2
pandas:               1.5.3
pyarrow:              15.0.1
pydantic:             2.6.3
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           1.4.52
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[MarcoGorelli]

hey - looks like you need to set a time zone to be able to use '%+'

In [36]: df['date'].cast(pl.Datetime('us', 'Europe/London')).dt.strftime('%+')
Out[36]:
shape: (3,)
Series: 'date' [str]
[
        "2021-04-01T01:00:00+01:00"
        "2021-04-02T01:00:00+01:00"
        "2021-04-03T01:00:00+01:00"
]

The error message for the Date one should be improved though

[marc-at-brightnight]

Thanks! Yes, using the below suits my use case. having a convenience function like df['date'].dt.isoformat() for the below might be something to think about.
df['date'].cast(pl.Datetime('us', 'UTC')).dt.strftime('%+')

Appreciate the help!!