HTTP Cache Freshness Calculator

• R1 Storage, RFC 9111 3: no-store means not stored; a shared cache does not store an unqualified private response ("The unqualified private response directive indicates that a shared cache MUST NOT store the response"); a shared cache does not store a response to a request with Authorization unless must-revalidate, public or s-maxage allows it (3.5); and the response must contain at least one of public, private (private cache only), Expires, max-age, s-maxage (shared cache only) or a heuristically cacheable status.
• R2 Heuristically cacheable statuses, RFC 9110 15.1: 200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414 and 501.
• R3 Freshness lifetime, RFC 9111 4.2.1, first match: "If the cache is shared and the s-maxage response directive (Section 5.2.2.10) is present, use its value, or If the max-age response directive (Section 5.2.2.1) is present, use its value, or If the Expires response header field (Section 5.3) is present, use its value minus the value of the Date response header field". Without Date, the time the response was received is used.
• R4 Expires, RFC 9111 5.3: "If a response includes a Cache-Control header field with the max-age directive (Section 5.2.2.1), a recipient MUST ignore the Expires header field." A shared cache also ignores it when s-maxage is present. "A cache recipient MUST interpret invalid date formats, especially the value "0", as representing a time in the past (i.e., "already expired")."
• R5 Heuristic, RFC 9111 4.2.2: "A cache MUST NOT use heuristics to determine freshness when an explicit expiration time is present in the stored response." With Last-Modified, the page multiplies your fraction by the time between Last-Modified and Date. "This specification does not provide specific algorithms, but it does impose worst-case constraints on their results." Without Last-Modified the value is cache-specific and nothing is computed.
• R6 Age, RFC 9111 4.2.3: apparent_age = max(0, response_time - date_value); response_delay = response_time - request_time; corrected_age_value = age_value + response_delay; corrected_initial_age = max(apparent_age, corrected_age_value), the conservative form, or "The corrected_age_value MAY be used as the corrected_initial_age"; resident_time = now - response_time; current_age = corrected_initial_age + resident_time. Then response_is_fresh = (freshness_lifetime > current_age), so a lifetime equal to the age is stale. A list-valued Age uses its first member (5.1).
• R7 no-cache, RFC 9111 5.2.2.4: the response may be stored, but it "MUST NOT be used to satisfy any other request without forwarding it for validation and receiving a successful response". The qualified form no-cache="field" is, per the RFC's own note, often handled as if it were unqualified.
• R8 Duplicates and conflicts, RFC 9111 4.2.1: for a repeated max-age or Expires, "either the first occurrence should be used or the response should be considered stale." Both readings are shown. Conflicting directives: the most restrictive is honored. A max-age that is not an integer: caches are encouraged to treat the response as stale.
• R9 delta-seconds overflow, RFC 9111 1.2.2: "the cache MUST consider the value to be 2147483648 (2^31) or the greatest positive integer it can conveniently represent." Larger values are clamped and flagged.
• R10 CDN column, RFC 9213: a targeted field is a Structured Fields Dictionary (RFC 8941 3.2). "If a targeted field in a given response is empty, or a parsing error is encountered, that field MUST be ignored by the cache". Otherwise the cache "MUST select the first (in target-list order) field with a valid, non-empty value and use its value to determine the caching policy for the response, and it MUST ignore the Cache-Control and Expires header fields in that response". "In particular, no-store and no-cache make max-age inoperative, and unrecognized extension directives are ignored." A decimal max-age is not coerced. With no usable targeted field, the CDN column equals the shared column.
• R11 Dates, RFC 9111 4.2: matched case-insensitively, all three HTTP-date formats accepted, a zone other than GMT treated as invalid for expiration, local time zones never used.

Recognised but not computed: stale-while-revalidate and stale-if-error, must-understand, Vary (listed only), 304 responses, request directives and the Pragma field. This is an independent tool, not affiliated with the IETF or any CDN vendor.