If-Unmodified-Since Inspect

Analyze cache-related headers across layers. No input is sent to a server. Use it for first-pass revalidation and CDN mismatch troubleshooting.

Status

Runs in your browser. No input is sent to a server. Use this as a first-pass diagnostic step.

How to use

Paste If-Unmodified-Since or Request Headers and click “Parse”. It renders the date clearly.

Notes (this tool)

  • Accepts If-Unmodified-Since: header lines (multi-line paste is OK).

About this page

What does this tool do?

Parse If-Unmodified-Since and show the date in UTC/local time.

Useful for conflict checks (preventing overwrites).

Basics (role of If-Unmodified-Since)

  • If-Unmodified-Since executes only if not modified since the given date.
  • If the condition fails, 412 Precondition Failed may be returned.
  • It pairs with Last-Modified.

Input examples

  • If-Unmodified-Since: Wed, 21 Oct 2015 07:28:00 GMT
  • Paste full Request Headers

Common pitfalls

  • Missing Last-Modified prevents condition check
  • Clock skew causes 412
  • Confusion when combined with If-Match

Debugging workflow (recommended)

  • Check value with Last-Modified Inspect
  • Parse If-Unmodified-Since here
  • Check if used with ETag/If-Match
  • Last-Modified Inspect
  • If-Modified-Since Inspect
  • ETag Inspect

Recommendations (practical)

  • For updates, combine If-Match/If-Unmodified-Since
  • Fetch latest state before update to avoid conflicts
  • Ensure Last-Modified accuracy and update timing

What this tool does

  • Summarize If-Unmodified-Since date
  • Highlight 412-related checks

Operational notes

  • Cache behavior changes across browser, CDN, and proxy layers, so compare captures from the same observation point.
  • Header-only diagnosis may be insufficient. Also review application cache invalidation strategy and key design.

Referenced specs

  • RFC 9110 (HTTP Semantics)
  • MDN: If-Unmodified-Since

FAQ

Difference between If-Unmodified-Since and If-Modified-Since?

If-Unmodified-Since runs only if unchanged; If-Modified-Since fetches only if changed.

References

  1. RFC 9110
  2. MDN: If-Unmodified-Since

These links are generated from site_map rules in recommended diagnostic order.

  1. Cache Validator Overview — Summarize relationships among ETag/Last-Modified validators
  2. If-None-Match Inspect — Parse If-None-Match and inspect revalidation conditions
  3. If-Match Inspect — Parse If-Match and inspect update preconditions
  4. If-Modified-Since Inspect — Parse If-Modified-Since and inspect conditional retrieval
  5. ETag Inspect — Parse ETag and If-None-Match consistency
  6. ETag Builder — Build ETag values for testing and operations
  7. Last-Modified Inspect — Parse Last-Modified and If-Modified-Since

Cache Validators

Connect ETag/Last-Modified and If-* to judge revalidation flow