LOGBACK-116: RollingCalendarTest fails when user.timezone is not CEST

Correct time-zone offset handling.

I caught this error at 2014/7/14 23:07 JST.
You'll reproduce by setting this: mvn test -DargLine="-Duser.timezone=Asia/Tokyo"
Test will pass when mvn test -DargLine="-Duser.timezone=CEST"

```
Running ch.qos.logback.core.rolling.helper.RollingCalendarTest
Tests run: 3, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.082 sec <<< FAILURE! - in ch.qos.logback.core.rolling.helper.RollingCalendarTest
testPeriodicity(ch.qos.logback.core.rolling.helper.RollingCalendarTest)  Time elapsed: 0.019 sec
testVaryingNumberOfHourlyPeriods(ch.qos.logback.core.rolling.helper.RollingCalendarTest)  Time elapsed: 0.006 sec
testVaryingNumberOfDailyPeriods(ch.qos.logback.core.rolling.helper.RollingCalendarTest)  Time elapsed: 0.008 sec  <<< FAILURE!
junit.framework.AssertionFailedError: p=20 expected:<1224946800000> but was:<1225033200000>
	at junit.framework.Assert.fail(Assert.java:50)
	at junit.framework.Assert.failNotEquals(Assert.java:287)
	at junit.framework.Assert.assertEquals(Assert.java:67)
	at junit.framework.Assert.assertEquals(Assert.java:134)
	at ch.qos.logback.core.rolling.helper.RollingCalendarTest.testVaryingNumberOfDailyPeriods(RollingCalendarTest.java:92)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at junit.framework.TestCase.runTest(TestCase.java:168)
	at junit.framework.TestCase.runBare(TestCase.java:134)
	at junit.framework.TestResult$1.protect(TestResult.java:110)
	at junit.framework.TestResult.runProtected(TestResult.java:128)
	at junit.framework.TestResult.run(TestResult.java:113)
	at junit.framework.TestCase.run(TestCase.java:124)
	at junit.framework.TestSuite.runTest(TestSuite.java:243)
	at junit.framework.TestSuite.run(TestSuite.java:238)
	at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:83)
	at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:264)
	at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:153)
	at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:124)
	at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:200)
	at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:153)
	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:103)


Results :

Failed tests: 
  RollingCalendarTest.testVaryingNumberOfDailyPeriods:92 p=20 expected:<1224946800000> but was:<1225033200000>
```