Java: Add a query for suspicious date format patterns.

java/ql/src/Likely Bugs/Likely Typos/SuspiciousDateFormat.java

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+System.out.println(new SimpleDateFormat("YYYY-MM-dd").format(new Date()));

java/ql/src/Likely Bugs/Likely Typos/SuspiciousDateFormat.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+
+<overview>
+<p>
+Some <code>SimpleDateFormat</code> patterns might not work correctly at the end of the calendar
+year, due to use of the <code>Y</code> placeholder (representing the ISO 8601 week year), rather
+than <code>y</code> representing the actual year.
+</p>
+</overview>
+
+<recommendation>
+<p>
+  Ensure the format pattern's use of <code>Y</code> is correct, and if not replace it with <code>y</code>.
+</p>
+</recommendation>
+
+<example>
+<p>
+  The following example uses the date format <code>YYYY-MM-dd</code>.
+  On the 30th of December 2019, this code will output "2020-12-30", rather than the intended "2019-12-30".
+</p>
+<sample src="SuspiciousDateFormat.java" />
+</example>
+
+<references>
+<li>
+  Java Platform, Standard Edition 7, API Specification:
+  <a href="https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html">SimpleDateFormat</a>.
+</li>
+</references>
+
+</qhelp>

java/ql/src/Likely Bugs/Likely Typos/SuspiciousDateFormat.ql

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+/**
+ * @name Suspicious date format
+ * @description Some date format patterns don't work as they might seem.
+ * @kind problem
+ * @problem.severity warning
+ * @precision high
+ * @id java/suspicious-date-format
+ * @tags correctness
+ */
+
+import java
+
+from ConstructorCall c, string format
+where
+  c.getConstructedType().hasQualifiedName("java.text", "SimpleDateFormat") and
+  format = c.getArgument(0).(StringLiteral).getValue() and
+  format.matches("%Y%") and
+  format.matches("%M%")
+select c, "Date formatter is passed a suspicious pattern \"" + format + "\"."