WebStorm IDE Date Conversion vs Node.js Time Zone Database

Direct answer

The inline formatting hints shown by WebStorm diverge from Node.js runtime date conversion primarily because the IDE and the Node.js process use different versions of the ICU time zone database, and WebStorm does not inherit the Node.js process’ TZ environment or system time‑zone setting.

Likely explanation

When a date string lacks an explicit offset, both WebStorm’s language service and Node.js rely on the host time zone to interpret the string. If the ICU data underlying the Intl API differs between the two, the same string can resolve to different offsets, producing non‑deterministic results.

Confirmed facts

• WebStorm bundles its own ICU data with the embedded Node.js/V8 engine that powers its JavaScript language service.
• A standalone Node.js installation can be built with small-icu, full-icu, or linked to the system ICU library; the Intl API’s time zone resolution follows whichever ICU version is present.
• Node.js updates its ICU data with each major release (e.g., Node.js 20 ships ICU ≈ 73.x), whereas WebStorm updates its bundled ICU only with IDE major releases, creating a version skew.
• The IDE’s language service does not read the process.env.TZ variable or the system time zone that a Node.js process sees, so its hints remain static unless the IDE itself is restarted.

Steps to align WebStorm hints with Node.js behavior

1. Check ICU versions: Open a terminal inside WebStorm and run

   node -p "process.versions.icu"

   . Note the version.
2. Compare with your project’s Node.js: In a system terminal running the same Node.js version used by the project, run the same command.
3. If versions match: Ensure the TZ environment variable is set identically in both environments (or unset to use the system zone). Restart WebStorm after changing TZ.
4. If versions differ: Upgrade WebStorm to the latest patch (which updates its bundled ICU) or align the project’s Node.js version to the one bundled with the IDE. Alternatively, rebuild Node.js with full-icu pointing to the same ICU data used by WebStorm.
5. Write deterministic code: Always supply an explicit time zone offset or IANA identifier when parsing or formatting dates, e.g.,

   new Date('2023-07-01T12:00:00Z')

   or

   new Intl.DateTimeFormat(undefined, {timeZone: 'America/New_York'}).format(date)

   . Avoid relying on implicit zone detection.

Diagnostic question (only needed if it changes the recommendation)

What ICU version does your project’s Node.js runtime report (node -p "process.versions.icu")? If it differs from the version shown in WebStorm’s built‑in terminal, follow the version‑alignment steps above; otherwise, focus on matching the TZ environment and using explicit zone identifiers.