Timezone can be set on TomlWriter.

DateConverterJdk6 class handles Java 6 date formatting limitations.

src/main/java/com/moandjiezana/toml/DateConverter.java

@@ -3,6 +3,7 @@ package com.moandjiezana.toml;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
 import java.util.Date;
+import java.util.TimeZone;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -10,6 +11,7 @@ import java.util.regex.Pattern;
 class DateConverter implements ValueConverter, ValueWriter {
 
   static final DateConverter DATE_PARSER = new DateConverter();
+  static final DateConverter DATE_PARSER_JDK_6 = new DateConverterJdk6();
   private static final Pattern DATE_REGEX = Pattern.compile("(\\d{4}-[0-1][0-9]-[0-3][0-9]T[0-2][0-9]:[0-5][0-9]:[0-5][0-9])(\\.\\d*)?(Z|(?:[+\\-]\\d{2}:\\d{2}))(.*)");
 
   @Override
@@ -89,16 +91,46 @@ class DateConverter implements ValueConverter, ValueWriter {
 
   @Override
   public void write(Object value, WriterContext context) {
-    DateFormat dateFormat = context.getDateFormat();
-    context.write(dateFormat.format(value));
+    DateFormat formatter = getFormatter(context.getTimeZone());
+    context.write(formatter.format(value));
   }
 
   @Override
   public boolean isPrimitiveType() {
     return true;
   }
-
+  
+  private DateFormat getFormatter(TimeZone timeZone) {
+    String format = "UTC".equals(timeZone.getID()) ? "yyyy-MM-dd'T'HH:m:ss'Z'" : "yyyy-MM-dd'T'HH:m:ssXXX";
+    SimpleDateFormat formatter = new SimpleDateFormat(format);
+    formatter.setTimeZone(timeZone);
+    
+    return formatter;
+  }
+  
   private DateConverter() {}
+  
+  private static class DateConverterJdk6 extends DateConverter {
+    @Override
+    public void write(Object value, WriterContext context) {
+      TimeZone timeZone = context.getTimeZone();
+      DateFormat formatter = getFormatter(timeZone);
+      String date = formatter.format(value);
+      
+      if ("UTC".equals(timeZone.getID())) {
+        context.write(date);
+      } else {
+        context.write(date.substring(0, 22)).write(':').write(date.substring(22));
+      }
+    }
+
+    private DateFormat getFormatter(TimeZone timeZone) {
+      String format = "UTC".equals(timeZone.getID()) ? "yyyy-MM-dd'T'HH:m:ss'Z'" : "yyyy-MM-dd'T'HH:m:ssZ";
+      SimpleDateFormat formatter = new SimpleDateFormat(format);
+      formatter.setTimeZone(timeZone);
+      return formatter;
+    }
+  }
 
   @Override
   public String toString() {

src/main/java/com/moandjiezana/toml/TomlWriter.java

@@ -50,6 +50,12 @@ public class TomlWriter {
       return this;
     }
     
+    public TomlWriter.Builder timeZone(TimeZone timeZone) {
+      this.timeZone = timeZone;
+      
+      return this;
+    }
+    
     /**
      * @param spaces number of spaces to put between opening square bracket and first item and between closing square bracket and last item
      * @return this TomlWriter.Builder instance

src/main/java/com/moandjiezana/toml/ValueWriters.java

@@ -2,6 +2,7 @@ package com.moandjiezana.toml;
 
 import static com.moandjiezana.toml.BooleanConverter.BOOLEAN_PARSER;
 import static com.moandjiezana.toml.DateConverter.DATE_PARSER;
+import static com.moandjiezana.toml.DateConverter.DATE_PARSER_JDK_6;
 import static com.moandjiezana.toml.MapValueWriter.MAP_VALUE_WRITER;
 import static com.moandjiezana.toml.NumberConverter.NUMBER_PARSER;
 import static com.moandjiezana.toml.ObjectValueWriter.OBJECT_VALUE_WRITER;
@@ -28,9 +29,13 @@ class ValueWriters {
   }
 
   private ValueWriters() {}
+  
+  private static DateConverter getPlatformSpecificDateConverter() {
+    return Runtime.class.getPackage().getSpecificationVersion().startsWith("1.6") ? DATE_PARSER_JDK_6 : DATE_PARSER;
+  }
 
   private static final ValueWriter[] VALUE_WRITERs = {
-      STRING_PARSER, NUMBER_PARSER, BOOLEAN_PARSER, DATE_PARSER,
+      STRING_PARSER, NUMBER_PARSER, BOOLEAN_PARSER, getPlatformSpecificDateConverter(),
       MAP_VALUE_WRITER, PRIMITIVE_ARRAY_VALUE_WRITER, TABLE_ARRAY_VALUE_WRITER
   };
 }

src/main/java/com/moandjiezana/toml/WriterContext.java

@@ -2,8 +2,6 @@ package com.moandjiezana.toml;
 
 import java.io.IOException;
 import java.io.Writer;
-import java.text.DateFormat;
-import java.text.SimpleDateFormat;
 import java.util.Arrays;
 import java.util.TimeZone;
 
@@ -108,12 +106,8 @@ class WriterContext {
     }
   }
   
-  DateFormat getDateFormat() {
-    String format = "yyyy-MM-dd'T'HH:m:ss'Z'";
-    SimpleDateFormat formatter = new SimpleDateFormat(format);
-    formatter.setTimeZone(timeZone);
-    
-    return formatter;
+  TimeZone getTimeZone() {
+    return timeZone;
   }
 
   WriterContext setIsArrayOfTable(boolean isArrayOfTable) {

src/test/java/com/moandjiezana/toml/TomlWriterTest.java

@@ -16,6 +16,7 @@ import java.net.URI;
 import java.net.URL;
 import java.util.Calendar;
 import java.util.Date;
+import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.LinkedList;
 import java.util.List;
@@ -350,6 +351,23 @@ public class TomlWriterTest {
     assertEquals(expected, new TomlWriter().write(new WithWrappers()));
   }
   
+  @Test
+  public void should_use_specified_time_zone() throws Exception {
+    Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
+    calendar.set(2015, Calendar.JULY, 1, 11, 15, 30);
+    calendar.set(Calendar.MILLISECOND, 0);
+    
+    Map<String, Date> o = new HashMap<String, Date>();
+    
+    o.put("sast", calendar.getTime());
+    
+    TomlWriter writer = new TomlWriter.Builder().
+      timeZone(TimeZone.getTimeZone("Africa/Johannesburg")).
+      build();
+    
+    assertEquals("sast = 2015-07-01T13:15:30+02:00\n", writer.write(o));
+  }
+  
   private static class SimpleTestClass {
     int a = 1;
   }