An administrator can set a specific visibility and publishing dates to make sure an asset will only be used in the intended period. You can find the (configured) permissions in the asset detail view under the planning tab.
The planning restrictions consist of two restrictions that can be set separately with the date-pickers:
Publish date - Downloading of the asset(s) will be limited to this date range.
The asset will only be available for 'publishing' within the date range it is set. What this actually does, is blocking the image from downloading, cropping and using it in a publication for all dates that are not included in the range you've set.
A blocked asset is still visible to all users, but it will have a little calendar icon with a cross in it in the asset manager overview. Hereby you can easily tell that this asset can't be used for publishing right now.
Visible date - Visibility of the asset(s) will be limited to this date range.
This setting will give you the possibility to hide assets. By default assets are always visible, so if you want assets to be hidden after a specific date, you have to set a visibility date.