This request header contains a timestamp of an existing fragment that is being reloaded.
The timestamp must be explicitly set by the user as an [up-time] attribute on the fragment.
It should indicate the time when the fragment's underlying data was last changed.
See [up-time] for a detailed example.
The time is encoded is the number of seconds elapsed since the Unix epoch.
For instance, a modification date of December 23th, 1:40:18 PM UTC would produce the following header:
X-Up-Target: .unread-count
X-Up-Reload-From-Time: 1608730818
If no timestamp is known, Unpoly will send a value of zero (X-Up-Reload-From-Time: 0).
@stable
