Added an ISO-8601 Timezone independent formatter

Server calendar had an issue with days going on
the wrong date if timezone was negative GMT.

Fixed the issue by implementing a timezone
independent ISO-8601 formatter to use with the dates on
the session calendar. The date given to ServerCalendar is already
set to midnight on each day.

Affects issues:
- Fixed #1239

Plan/common/src/main/java/com/djrapitops/plan/delivery/formatting/Formatters.java

@@ -43,6 +43,7 @@ public class Formatters {
     private final SecondFormatter secondLongFormatter;
     private final ClockFormatter clockLongFormatter;
     private final ISO8601NoClockFormatter iso8601NoClockLongFormatter;
+    private final ISO8601NoClockTZIndependentFormatter iso8601NoClockTZIndependentFormatter;
 
     private final TimeAmountFormatter timeAmountFormatter;
 
@@ -56,6 +57,7 @@ public class Formatters {
         clockLongFormatter = new ClockFormatter(config, locale);
         secondLongFormatter = new SecondFormatter(config, locale);
         iso8601NoClockLongFormatter = new ISO8601NoClockFormatter(config, locale);
+        iso8601NoClockTZIndependentFormatter = new ISO8601NoClockTZIndependentFormatter();
 
         yearFormatter = new DateHolderFormatter(yearLongFormatter);
         dayFormatter = new DateHolderFormatter(dayLongFormatter);
@@ -110,6 +112,10 @@ public class Formatters {
         return iso8601NoClockLongFormatter;
     }
 
+    public Formatter<Long> iso8601NoClockTZIndependentLong() {
+        return iso8601NoClockTZIndependentFormatter;
+    }
+
     public Formatter<Long> timeAmount() {
         return timeAmountFormatter;
     }

Plan/common/src/main/java/com/djrapitops/plan/delivery/formatting/time/ISO8601NoClockTZIndependentFormatter.java

@@ -0,0 +1,40 @@
+/*
+ *  This file is part of Player Analytics (Plan).
+ *
+ *  Plan is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Lesser General Public License v3 as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Plan is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with Plan. If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.djrapitops.plan.delivery.formatting.time;
+
+import com.djrapitops.plan.delivery.formatting.Formatter;
+
+import java.text.SimpleDateFormat;
+
+/**
+ * Formatter for a timestamp in ISO-8601 format without the clock, without applying timezone offset.
+ *
+ * @author Rsl1122
+ */
+public class ISO8601NoClockTZIndependentFormatter implements Formatter<Long> {
+
+    @Override
+    public String apply(Long date) {
+        return date > 0 ? format(date) : "-";
+    }
+
+    private String format(Long date) {
+        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
+
+        return dateFormat.format(date);
+    }
+}

Plan/common/src/main/java/com/djrapitops/plan/delivery/rendering/json/graphs/calendar/CalendarFactory.java

@@ -68,7 +68,7 @@ public class CalendarFactory {
     ) {
         return new ServerCalendar(
                 uniquePerDay, newPerDay, playtimePerDay, sessionsPerDay,
-                formatters.iso8601NoClockLong(), formatters.timeAmount(), theme, locale
+                formatters.iso8601NoClockTZIndependentLong(), formatters.timeAmount(), theme, locale
         );
     }
 }

Plan/common/src/main/java/com/djrapitops/plan/delivery/rendering/json/graphs/calendar/ServerCalendar.java

@@ -38,8 +38,8 @@ public class ServerCalendar {
     private final SortedMap<Long, Integer> sessionsPerDay;
     private final SortedMap<Long, Long> playtimePerDay;
 
-    private final Formatter<Long> iso8601Formatter;
+    private final Formatter<Long> iso8601TZIndependent;
-    private final Formatter<Long> timeAmountFormatter;
+    private final Formatter<Long> timeAmount;
     private final Theme theme;
     private final Locale locale;
 
@@ -48,15 +48,15 @@ public class ServerCalendar {
             SortedMap<Long, Integer> newPerDay,
             SortedMap<Long, Long> playtimePerDay,
             NavigableMap<Long, Integer> sessionsPerDay,
-            Formatter<Long> iso8601Formatter,
+            Formatter<Long> iso8601TZIndependent,
-            Formatter<Long> timeAmountFormatter,
+            Formatter<Long> timeAmount,
             Theme theme,
             Locale locale
     ) {
         this.uniquePerDay = uniquePerDay;
         this.newPerDay = newPerDay;
-        this.iso8601Formatter = iso8601Formatter;
+        this.iso8601TZIndependent = iso8601TZIndependent;
-        this.timeAmountFormatter = timeAmountFormatter;
+        this.timeAmount = timeAmount;
         this.sessionsPerDay = sessionsPerDay;
         this.playtimePerDay = playtimePerDay;
         this.theme = theme;
@@ -87,7 +87,7 @@ public class ServerCalendar {
             }
 
             Long key = entry.getKey();
-            String day = iso8601Formatter.apply(key);
+            String day = iso8601TZIndependent.apply(key);
 
             series.append(",{\"title\": \"").append(locale.get(HtmlLang.NEW_CALENDAR)).append(" ").append(newPlayers)
                     .append("\",\"start\":\"").append(day)
@@ -104,7 +104,7 @@ public class ServerCalendar {
             }
 
             Long key = entry.getKey();
-            String day = iso8601Formatter.apply(key);
+            String day = iso8601TZIndependent.apply(key);
 
             series.append(",{\"title\": \"").append(locale.get(HtmlLang.UNIQUE_CALENDAR)).append(" ").append(uniquePlayers)
                     .append("\",\"start\":\"").append(day)
@@ -120,9 +120,9 @@ public class ServerCalendar {
                 continue;
             }
             Long key = entry.getKey();
-            String day = iso8601Formatter.apply(key);
+            String day = iso8601TZIndependent.apply(key);
 
-            series.append(",{\"title\": \"").append(locale.get(HtmlLang.LABEL_PLAYTIME)).append(": ").append(timeAmountFormatter.apply(playtime))
+            series.append(",{\"title\": \"").append(locale.get(HtmlLang.LABEL_PLAYTIME)).append(": ").append(timeAmount.apply(playtime))
                     .append("\",\"start\":\"").append(day)
                     .append("\",\"color\": \"").append(theme.getValue(ThemeVal.GREEN)).append('"')
                     .append("}");
@@ -136,7 +136,7 @@ public class ServerCalendar {
                 continue;
             }
             Long key = entry.getKey();
-            String day = iso8601Formatter.apply(key);
+            String day = iso8601TZIndependent.apply(key);
 
             series.append(",{\"title\": \"").append(locale.get(HtmlLang.SIDE_SESSIONS)).append(": ").append(sessionCount)
                     .append("\",\"start\":\"").append(day)