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

Notes (this tool)

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

About this page

What does this tool do?

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

Useful for diagnosing 304 Not Modified and cache validation.

Basics (role of If-Modified-Since)

  • If-Modified-Since requests the resource only if modified after the given date.
  • If not modified, 304 is returned; otherwise typically 200.
  • It pairs with Last-Modified.

Input examples

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

Common pitfalls

  • Missing Last-Modified makes IMS ineffective
  • Clock skew prevents 304
  • Behavior changes when combined with ETag (priority matters)

Debugging workflow (recommended)

  • Check value with Last-Modified Inspect
  • Parse If-Modified-Since here
  • Check ETag usage with ETag Inspect
  • Last-Modified Inspect
  • ETag Inspect
  • Response Headers Parser

Recommendations (practical)

  • Always pair with Last-Modified
  • Use with Cache-Control: no-cache for revalidation
  • Always include validators for APIs/HTML

What this tool does

  • Summarize If-Modified-Since date
  • Highlight 304-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-Modified-Since

FAQ

Difference between If-Modified-Since and If-None-Match?

If-None-Match uses ETags; If-Modified-Since uses dates.

References

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

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

  1. How to Diagnose Missing 304 Responses — Trace ETag/Last-Modified and If-* round trips to isolate missing 304 behavior
  2. If-None-Match Inspect — Parse If-None-Match and inspect revalidation conditions
  3. Age Inspect — Parse Age to estimate shared-cache residency time
  4. If-Unmodified-Since Inspect — Parse If-Unmodified-Since and inspect precondition behavior
  5. Expires Inspect — Parse Expires and Date to inspect freshness behavior
  6. Cache Validator Overview — Summarize relationships among ETag/Last-Modified validators
  7. ETag Inspect — Parse ETag and If-None-Match consistency
  8. ETag Builder — Build ETag values for testing and operations

Cache Validators

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