If-Unmodified-Since

The HTTP If-Unmodified-Since request header makes an HTTP request conditional. The server processes the request only when the target resource has not been modified after the specified date and time.

Table of Contents

Usage
Directives
- HTTP-date
Example
Takeaway
See also

Usage

Clients include If-Unmodified-Since to guard against overwriting changes made by others. The most common scenario involves an HTTP PUT or DELETE request where the client needs certainty the resource remains in the state observed earlier. If another client modified the resource after the date in the header, the server rejects the request with a 412 Precondition Failed response.

This addresses the lost update problem. Two clients fetch the same resource, both make edits, and both send updates. Without a precondition, the second update silently overwrites the first. Adding If-Unmodified-Since to the second request ensures the server detects the conflict.

The header value uses the HTTP date format. The server compares this date against the Last-Modified timestamp of the resource. When the resource modification date is equal to or earlier than the value in If-Unmodified-Since, the condition passes and the server continues processing.

For stronger consistency guarantees, prefer If-Match with an ETag value. ETags provide byte-level precision, while date-based conditions have a resolution of one second.

Directives

HTTP-date

The value follows the IMF-fixdate format. The date represents the latest acceptable modification time for the resource.

If-Unmodified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT

day-name: abbreviated weekday (Mon, Tue, Wed, Thu, Fri, Sat, Sun)
day: two-digit day of the month
month: abbreviated month (Jan, Feb, Mar, etc.)
year: four-digit year
hour:minute:second: time in 24-hour format
GMT: all HTTP dates use Greenwich Mean Time

Example

A client fetched a document on February 15, 2025, and now sends an update. The If-Unmodified-Since value matches the Last-Modified date received in the original response. If the document changed after this timestamp, the server responds with 412 Precondition Failed.

PUT /docs/report.json HTTP/1.1
Host: api.example.re
Content-Type: application/json
If-Unmodified-Since: Sat, 15 Feb 2025 10:30:00 GMT

When paired with a Range request for partial uploads, If-Unmodified-Since ensures the base resource has not changed between chunks.

PATCH /files/dataset.csv HTTP/1.1
Host: api.example.re
If-Unmodified-Since: Fri, 14 Feb 2025 08:00:00 GMT
Content-Range: bytes 0-1023/4096

Takeaway

The If-Unmodified-Since header prevents accidental overwrites by conditioning the request on the resource remaining unchanged since a specific date. The server rejects the request with 412 when a newer version exists.