TeX \time primitive: missing time zone configuration for reproducible timestamps

Answer

There is no built‑in mechanism in the core TeX engine to declare a time zone; the \\time, \\day, \\month, and \\year primitives are set once at startup from the C library localtime() call, which reflects the process’s local time zone.

Confirmed facts

• The primitives expand to integers derived from localtime() at engine start; there is no primitive to query, set, or offset the time zone.
• The \\pdfcreationdate primitive inherits the same local‑time value, so PDF metadata varies with the host TZ.
• Engines that support the \\pdfinfo primitive (pdfTeX, XeTeX, LuaTeX) allow the creation‑date string to be overridden manually.

Likely explanation

To obtain zone‑independent values you must fix the source of time before the engine starts, e.g., by setting the TZ environment variable to UTC, or, in LuaTeX, compute UTC via os.date('!*t') and redefine the primitives with \\directlua.

Steps for reproducible timestamps

1. If you can control the invocation environment, export TZ=UTC before running the engine:

export TZ=UTC
pdflatex mydoc.tex

2. If you are using LuaTeX and prefer not to alter the environment, add to the preamble:

\\directlua{
  local utc = os.date('!*t')
  tex.time = utc.hour*60 + utc.min
  tex.day = utc.day
  tex.month = utc.month
  tex.year = utc.year
}

This forces the primitives to return UTC values for the rest of the run.

Fixed UTC PDF creation date

Without touching the environment, define a fixed timestamp string and assign it to the PDF Info dictionary:

\\def\\mycreationdate{D:20261012074125Z}
\\pdfinfo{/CreationDate (\\mycreationdate)}

This works with pdfTeX, XeTeX, or LuaTeX; engines lacking \\pdfinfo (plain TeX) cannot modify the creation date.

Verification

Check that \\time changes when TZ is toggled:

tex -halt-on-error '\\showthe\\time'
TZ=UTC tex -halt-on-error '\\showthe\\time'

Inspect the PDF’s Info field with pdfinfo or pdftk to confirm the CreationDate matches the supplied string.

Diagnostic note: Are you using pdfTeX, XeTeX, or LuaTeX? The manual \\pdfcreationdate override relies on the \\pdfinfo primitive, which is unavailable in plain TeX builds.