Sets the time when the fragment's underlying data was last changed.
When the fragment is reloaded,
its known modification time is sent as an If-Modified-Since request header.
The server may check the header and decide to skip rendering.
See Conditional requests for a full example.
How [up-etag] attributes are set
Unpoly will automatically set an [up-time] attribute when a fragment was rendered
from a response with a Last-Modified header. When a fragment was rendered without such a header,
Unpoly will set [up-time=false] to indicate that its modification time is unknown.
A large response may contain multiple fragments that are later reloaded individually
and should each have their own modification time. In this case the server may also also render multiple
fragments with each their own [up-time] attribute.
See Individual versions per fragment for an example.
