---
name: "If-Modified-Since"
area: "API"
kind: "HTTP header"
parent: "up.protocol module"
url: "https://unpoly.com/api/If-Modified-Since"
---
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# If-Modified-Since

This request header contains the last modification time of a fragment that is being reloaded.

The server can use the header value to prevent unnecessary re-rendering of unchanged content. See [Conditional requests](https://unpoly.com/learn/conditional-requests.md) for details and examples.

The header is only set when Unpoly knows the last modification time of the loading fragment. For this the fragment must have been rendered with an [`Last-Modified`](https://unpoly.com/api/Last-Modified.md) response header or [`[up-time]`](https://unpoly.com/api/up-time.md) attribute.

## Format {#format}

The header value is a date/time in [RFC 1123](https://www.rfc-editor.org/rfc/rfc1123) format:

```http
If-Modified-Since: Wed, 15 Nov 2000 13:11:22 GMT
```