BUG: tz_localize gives incorrect shift_forward value in some European timezones

[Liam3851]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample, a copy-pastable example

In [11]: pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/London', nonexistent='shift_forward')
Out[11]: Timestamp('2021-03-28 03:00:00+0100', tz='Europe/London')

In [12]: pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/London', nonexistent='shift_forward') - pd.Timedelta(60, 'm')
Out[12]: Timestamp('2021-03-28 02:00:00+0100', tz='Europe/London')

In [13]: pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/London', nonexistent='shift_forward') - pd.Timedelta(61, 'm')
Out[13]: Timestamp('2021-03-28 00:59:00+0000', tz='Europe/London')

Problem description

When attempting to localize a non-existent time in the Europe/London timezone using the nonexistent='shift_forward', parameter, the resulting time is 03:00 summer time-- even though the time transition skips from 01:00 AM UTC to 02:00 AM summer time.

The issue appears to occur in European timezones that are in UTC when the time is not summer time (i.e. London, Dublin, and Lisbon). Timezones that start at UTC+1 and have their summer time conversions from 2 AM-3 AM work properly.

In [23]: pd.to_datetime('2021-03-28 02:30').tz_localize('Europe/Paris', nonexistent='shift_forward')
Out[23]: Timestamp('2021-03-28 03:00:00+0200', tz='Europe/Paris')

In [24]: pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/Dublin', nonexistent='shift_forward')
Out[24]: Timestamp('2021-03-28 03:00:00+0100', tz='Europe/Dublin')

In [25]: pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/Lisbon', nonexistent='shift_forward')
Out[25]: Timestamp('2021-03-28 03:00:00+0100', tz='Europe/Lisbon')

In [26]: pd.to_datetime('2021-03-28 02:30').tz_localize('Europe/Paris', nonexistent='shift_forward')
Out[26]: Timestamp('2021-03-28 03:00:00+0200', tz='Europe/Paris')

In [27]: pd.to_datetime('2021-03-28 02:30').tz_localize('Europe/Stockholm', nonexistent='shift_forward')
Out[27]: Timestamp('2021-03-28 03:00:00+0200', tz='Europe/Stockholm')

Expected Output

Per the documentation localizing with nonexistent='shift_forward' should "shift the nonexistent time forward to the closest existing time". In this case the closest existing time in Europe/London to 2021-03-28 01:30 would be 2021-03-28 02:00+0100.

In [11]: pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/London', nonexistent='shift_forward')
Out[11]: Timestamp('2021-03-28 02:00:00+0100', tz='Europe/London')

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 7d4757b
python : 3.8.8.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19041
machine : AMD64
processor : Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United States.1252

pandas : 1.3.0.dev0+1297.g7d4757b4de
numpy : 1.20.2
pytz : 2021.1
dateutil : 2.8.1
pip : 21.0.1
setuptools : 49.6.0.post20210108
Cython : 0.29.22
pytest : 6.2.2
hypothesis : 6.8.3
sphinx : 3.5.3
blosc : None
feather : None
xlsxwriter : 1.3.8
lxml.etree : 4.6.3
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.3
IPython : 7.22.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 0.8.7
fastparquet : 0.5.0
gcsfs : None
matplotlib : 3.4.0
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : 3.0.0
pyxlsb : None
s3fs : 0.5.2
scipy : 1.6.2
sqlalchemy : 1.4.3
tables : 3.6.1
tabulate : 0.8.9
xarray : 0.17.0
xlrd : 2.0.1
xlwt : 1.3.0
numba : 0.53.1

[mroeschke]

Looks like there's Timestamp constructor differences as shown here:

In [2]: ts = pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/London', nonexistent='shift_forward')

In [3]: ts
Out[3]: Timestamp('2021-03-28 03:00:00+0100', tz='Europe/London')

In [4]: pd.Timestamp(ts.value, tz=ts.tz)
Out[4]: Timestamp('2021-03-28 03:00:00+0100', tz='Europe/London')

In [5]: pd.Timestamp(ts.value).tz_localize(ts.tz)
Out[5]: Timestamp('2021-03-28 02:00:00+0100', tz='Europe/London')

In [6]: pd.__version__
Out[6]: '1.3.0.dev0+1297.g7d4757b4de'

[mroeschke]

Tracked down the issue to this line in _localize_tso

pandas/pandas/_libs/tslibs/conversion.pyx

Line 737 in 7d4757b

pos = trans.searchsorted(obj.value, side='right') - 1

in which for the 'shift_forward' we actually want the right edge index

# debugging git diff
+            print(obj.dts.hour)
             dt64_to_dtstruct(obj.value + deltas[pos], &obj.dts)
+            print(obj.dts.hour)

In [1]: ts = pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/London', nonexistent='shift_forward')
2
3

[thomie]

This issue seems to be fixed (version 2.3.1).

>>> pd.to_datetime('2021-03-28 01:30').tz_localize('Europe/London', nonexistent='shift_forward')
Timestamp('2021-03-28 02:00:00+0100', tz='Europe/London')

Output of pd.show_versions()

Details

INSTALLED VERSIONS

commit : c888af6
python : 3.12.11
python-bits : 64
OS : Linux
OS-release : 6.12.41
Version : #1-NixOS SMP PREEMPT_DYNAMIC Fri Aug 1 08:48:47 UTC 2025
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.3.1
numpy : 2.3.1
pytz : 2025.2
dateutil : 2.9.0.post0
pip : None
Cython : None
sphinx : None
IPython : 9.4.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : None
lxml.etree : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 20.0.0
pyreadstat : None
pytest : 8.4.1
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : 2.0.42
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None