diff --git a/pyproject.toml b/pyproject.toml
index 2eb1716b3..674bbe020 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -144,6 +144,10 @@ dev = [
     "pytest-asyncio>=0.21.0",
     "pytest-cov>=4.0.0",
     "pytest-xdist>=3.8,<4",
+    # zoneinfo falls back to IANA tzdata when the OS ships no zone database or
+    # drops the legacy aliases (ubuntu-26.04 runners lack ``US/Pacific``); the
+    # reporting period-date tests resolve those aliases.
+    "tzdata==2026.6",
     "freezegun==1.5.5",  # isolated deferred-schema compatibility regressions
     "mypy==1.20.2",
     "pyright==1.1.414",  # real imported-base adopter regressions
@@ -446,6 +450,7 @@ dev = [
     # source — and a dead-weight hook that everyone bypasses with
     # ``SKIP=mypy``.
     "mypy==1.20.2",
+    "tzdata==2026.6",
 ]
 
 [[tool.mypy.overrides]]