datetime2 and babel: handling regional time zones when switching languages

Answer

As of datetime2 v2.XX (released 2023) the package does **not** provide a built‑in mechanism to change the time‑zone offset when the document language is switched with babel. The offset is fixed at package load time and can only be altered manually via \\DTMsetoffset. Consequently, datetime2 does not expose a \\DTMsettimezone command, nor does it derive offsets from IANA zone identifiers or apply DST rules automatically.

Why the package behaves this way

• datetime2’s primary purpose is formatting dates and times according to language‑specific patterns; it stores a simple numeric offset (minutes east of UTC) that is set once when the package is loaded.
• babel’s language switching hooks (\\extras) affect only the strings used for month names, day names, etc.; they do not touch the internal offset variable.
• Implementing IANA‑zone support would require a substantial rewrite (parsing zoneinfo data, handling historic DST rules) which is outside the scope of datetime2.

How to add the desired behaviour yourself

1. Define a command that updates the offset, e.g.

   \\newcommand{\\DTMsettimezone}[1]{%
     % #1 = IANA zone identifier, e.g. Europe/Paris
     \\ifdefined\\zones@offset
       % placeholder: you would need to compute the offset from #1
       % For demonstration we assume a macro \\zoneoffset{} returns minutes
       \\DTMsetoffset{\\zoneoffset{#1}}%
     \\else
       \\PackageWarning{datetime2}{No zone‑offset lookup available}%
     \\fi
   }

2. Hook the command into babel’s language extras so it runs automatically after a language change:

   \\addto\\extras{%
     \\DTMsettimezone{}%
   }

   Replace with the babel language identifier (e.g. english, french) and with the zone you want for that language (e.g. Europe/London for english, Europe/Paris for french).
3. If you prefer a manual approach, simply call \\DTMsettimezone{} whenever you need to change the offset.

Verification

Compile a minimal example that prints the current time before and after a language switch:

\\documentclass{article}
\\usepackage[english,french]{babel}
\\usepackage{datetime2}
\\DTMsetstyle{iso}
\\begin{document}
\\DTMnow% shows time with initial offset
\\selectlanguage{french}
\\DTMnow% same offset unless you called \\DTMsettimezone
\\end{document}

Observe that the hour/minute part does not change after switching to French unless you insert the appropriate \\DTMsettimezone call in the extrasfrench hook.