Test build: JDK 10 build 41
Test env: Mac 10.13 with ar_SA locale.

Error message:
----------System.err:(27/2508)*----------
 stdout: [jar signed.

Warning:
The signer's certificate is self-signed.
No -tsa or -tsacert is provided and this jar is not timestamped. Without a timestamp, users may not be able to validate this jar after the signer certificate's expiration date (2019-01-25).
];
 stderr: []
 exitValue = 0

java.lang.RuntimeException: 'No -tsa or -tsacert is provided and this jar is not timestamped. Without a timestamp, users may not be able to validate this jar after the signer certificate's expiration date (\u0662\u0660\u0661\u0669-\u0660\u0661-\u0662\u0665).' missing from stdout/stderr

at jdk.testlibrary.OutputAnalyzer.shouldContain(OutputAnalyzer.java:106)
at Test.checkSigning(Test.java:176)
at NoTimestampTest.start(NoTimestampTest.java:83)
at NoTimestampTest.main(NoTimestampTest.java:44)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:564)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:115)
at java.base/java.lang.Thread.run(Thread.java:844)

JavaTest Message: Test threw exception: java.lang.RuntimeException: 'No -tsa or -tsacert is provided and this jar is not timestamped. Without a timestamp, users may not be able to validate this jar after the signer certificate's expiration date (\u0662\u0660\u0661\u0669-\u0660\u0661-\u0662\u0665).' missing from stdout/stderr

The root cause:
L81: String warning = String.format(NO_TIMESTAMP_SIGNING_WARN_TEMPLATE,
                expirationDate);
L95: warning = String.format(NO_TIMESTAMP_VERIFYING_WARN_TEMPLATE, expirationDate);

String.format(String format, Object... args) use system default locale which is ar_SA to format the string, so displaying the date using ARABIC-INDIC digits(\u0660~\u0669), not (0~9).
Suggest to use another method: String.format​(Locale l, String format, Object... args) to specify locale to en_US.