Add PD_YEAR constant for use across the site

lib/Constants.php

@@ -32,6 +32,8 @@ const SOURCE_PROJECT_GUTENBERG_AUSTRALIA = 8;
 
 const AVERAGE_READING_WORDS_PER_MINUTE = 275;
 
+define('PD_YEAR', intval(gmdate('Y')) - 96);
+
 // No trailing slash on any of the below constants.
 const SITE_URL =			'https://standardebooks.org';
 const SITE_ROOT =			'/standardebooks.org';

www/about/index.php

@@ -57,7 +57,7 @@ require_once('Core.php');
 			<p>The public domain is a priceless resource for all of us, and for the generations after us. It’s a free repository of our culture going back centuries—a way for us to see where we came from and to chart where we’re going. It represents our collective cultural heritage.</p>
 			<p>In the past, copyright was a limited boon, designed not to enrich a creator and their children’s children a hundred years from now, but rather to allow a creator to profit by granting a <em>temporary</em> monopoly on reproduction, in exchange for their work to be returned to the public after a few years. Our ancestors—in fact, the framers of the US Constitution—recognized that art builds on art, and that locking up culture benefits a handful but harms the greater public.</p>
 			<p>Today, large corporations are putting a lot of money into twisting our laws to slowly but surely strangle the public domain, making it increasingly remote and inaccessible so they can continue seeking rent on ideas and culture nearly a century old. Today laws lock up work not just for the author’s entire lifetime, but for the lifetime of their children, and <em>their</em> children. Copyright can’t enrich the dead, but it <em>can</em> enrich powerful corporations … at our—at <em>everyone’s</em>—expense.</p>
-			<p>Dedicating the work Standard Ebooks produces to the public domain is our small way of letting the world know how important it is to fight that. If corporations have their way, the last liberated and free culture you’ll ever have is what was published before 1924.</p>
+			<p>Dedicating the work Standard Ebooks produces to the public domain is our small way of letting the world know how important it is to fight that. If corporations have their way, the last liberated and free culture you’ll ever have is what was published before <?= PD_YEAR ?>.</p>
 			<p>What a sad world that would be.</p>
 		</section>
 	</article>

www/contribute/accepted-ebooks.php

@@ -4,7 +4,7 @@ require_once('Core.php');
 <main>
 	<article>
 		<h1>Ebooks We Do and Don’t Accept</h1>
-		<p>Standard Ebooks only works on books that have entered the U.S. public domain due to copyright expiration.</p>
+		<p>Standard Ebooks only works on books that have entered the U.S. public domain due to copyright expiration. Generally this means a book must have been published in <?= PD_YEAR ?> or earlier, though there are exceptions for works from later periods that did not follow copyright formalities. For full details, see <a href="https://www.gutenberg.org/help/copyright.html">Project Gutenberg’s Copyright How-To</a>.</p>
 		<h2>Types of ebooks we do accept</h2>
 		<ul>
 			<li>

www/contribute/producing-an-ebook-step-by-step.php

@@ -47,12 +47,12 @@ require_once('Core.php');
 						<p><a href="https://books.google.com">Google Books</a></p>
 					</li>
 				</ul>
-				<p>Each of those sources allows you to filter results by publication date, so make sure you select 1924 and earlier to ensure they’re in the US public domain.</p>
+				<p>Each of those sources allows you to filter results by publication date, so make sure you select <?= PD_YEAR ?> and earlier to ensure they’re in the U.S. public domain.</p>
 				<p>If you can’t find scans of your book at the above sources, and you’re using a Project Gutenberg transcription as source material, there’s a good chance that PGDP (the sister project of Project Gutenberg that does the actual transcriptions) <a href="https://www.pgdp.org/ols/">has a copy of the scans they used accessible in their archives</a>. You should only use the PGDP archives as a last resort; because their scans are not searchable, verifying typos becomes extremely time-consuming.</p>
 				<p>Please keep the following important notes in mind when searching for page scans:</p>
 				<ul>
 					<li>
-						<p>Make sure the scans you find are <em>published in 1924 or earlier.</em> You <em>must verify the copyright page in the page scans</em> before proceeding.</p>
+						<p>Make sure the scans you find are <em>published in <?= PD_YEAR ?> or earlier.</em> You <em>must verify the copyright page in the page scans</em> before proceeding.</p>
 					</li>
 					<li>
 						<p>Often you’ll find different editions, published at different times by different publishers, for the same book. It’s worth the effort to quickly browse through each different one to get an idea of the kinds of changes the different publishers introduced. Maybe one edition is better than another!</p>
@@ -505,7 +505,7 @@ proceed to seal up my confession, I bring the life of that unhappy Henry Jekyll
 						<p>The image you pick should be a “fine art” oil painting so that all Standard Ebooks have a consistent cover style. This is actually easier than you think, because it turns out most public domain artwork is from the era of fine art.</p>
 					</li>
 					<li>
-						<p>You must provide proof of public domain status to the SE Editor-in-Chief in the form of a page scan of the painting from a 1924-or-older book, and the Editor-in-Chief must approve your selection before you can commit it to your repository.</p>
+						<p>You must provide proof of public domain status to the SE Editor-in-Chief in the form of a page scan of the painting from a <?= PD_YEAR ?>-or-older book, and the Editor-in-Chief must approve your selection before you can commit it to your repository.</p>
 					</li>
 					<li>
 						<p>The Standard Ebooks Editor-in-Chief has the final say on the cover image you pick, and it may be rejected for, among other things, poor public domain status research, being too low resolution, not fitting in with the “fine art” style, or being otherwise inappropriate for your ebook.</p>

www/contribute/uncategorized-art-resources.php

@@ -4,7 +4,7 @@ require_once('Core.php');
 <main>
 	<article id="a-basic-standard-ebooks-source-folder">
 		<h1>Uncategorized art books</h1>
-		<p>You may find these 1924-and-earlier art books useful as you conduct cover art research.</p>
+		<p>You may find these <?= PD_YEAR ?>-and-earlier art books useful as you conduct cover art research.</p>
 		<ul>
 			<li><a href="https://books.google.com/books?id=PBpAAAAAYAAJ">The History of Modern Painting</a></li>
 			<li><a href="https://books.google.com/books?id=Q3YoAAAAYAAJ">Art Studies for Schools: Or, Hints on the Use of Reproductions of High Art</a></li>