diff --git a/phpunit.xml b/phpunit.xml
index 4d37631..78aa5f8 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -3,5 +3,8 @@
         <testsuite name="Models">
             <directory>tests/Models</directory>
         </testsuite>
+        <testsuite name="Other">
+            <file>tests/TimeKeeperTest.php</file>
+        </testsuite>
     </testsuites>
 </phpunit>
diff --git a/src/TimeKeeper.php b/src/TimeKeeper.php
new file mode 100644
index 0000000..cc4a1e8
--- /dev/null
+++ b/src/TimeKeeper.php
@@ -0,0 +1,105 @@
+<?php
+declare(strict_types=1);
+
+namespace LotGD\Core;
+
+class TimeKeeper {
+  private $adjustedEpoch;
+  private $theBeginning;
+
+  private $secondsPerMinute = 60;
+  private $secondsPerHour = 60 * 60;
+  private $secondsPerDay = 60 * 60 * 24;
+  private $secondsPerGameYear;
+  private $secondsPerGameDay;
+  private $secondsPerGameHour;
+  private $secondsPerGameMinute;
+  private $secondsPerGameSecond;
+
+  public function __construct(\DateTime $gameEpoch, int $gameOffsetSeconds, int $gameDaysPerDay) {
+    $gameEpochCopy = clone($gameEpoch);
+    $this->adjustedEpoch = $gameEpochCopy->add(
+      $this->interval(0, 0, 0, 0, $gameOffsetSeconds)
+    );
+    $this->theBeginning = new \DateTime("0000-01-01 UTC");
+
+    $this->secondsPerGameDay = (float) $this->secondsPerDay / $gameDaysPerDay;
+    $this->secondsPerGameYear = $this->secondsPerGameDay * 365;
+    $this->secondsPerGameHour = $this->secondsPerGameDay / 24;
+    $this->secondsPerGameMinute = $this->secondsPerGameHour / 60;
+    $this->secondsPerGameSecond = $this->secondsPerGameMinute / 60;
+  }
+
+  public function isNewDay(DateTime $lastInteractionTime): bool {
+    if ($lastInteractionTime == null) {
+      return true;
+    }
+    $t1 = $this->gameTime();
+    $t2 = $this->convertToGameTime($lastInteractionTime);
+    $d1 = $t1->format("Y-m-d");
+    $d2 = $t2->format("Y-m-d");
+
+    return $d1 != $d2;
+  }
+
+  public function gameTime(): \DateTime {
+    return $this->convertToGameTime(new \DateTime());
+  }
+
+  public function convertFromGameTime(
+    \DateTime $time
+  ): \DateTime {
+    // Game dates are in the distant past, better not use getTimestamp().
+    $i = $this->theBeginning->diff($time);
+
+    $seconds = 0;
+    $seconds += $i->days * $this->secondsPerGameDay;
+    $seconds += $i->h * $this->secondsPerGameHour;
+    $seconds += $i->i * $this->secondsPerGameMinute;
+    $seconds += $i->s * $this->secondsPerGameSecond;
+
+    $ret = clone($this->adjustedEpoch);
+    return $ret->add($this->interval(0, 0, 0, 0, (int) $seconds));
+  }
+
+  public function convertToGameTime(
+    \DateTime $time
+  ): \DateTime {
+    $timeUnix = $time->getTimestamp();
+    $epochUnix = $this->adjustedEpoch->getTimestamp();
+
+    $interval = $timeUnix - $epochUnix;
+
+    $years = (int) ($interval / $this->secondsPerGameYear);
+    $interval -= $years * $this->secondsPerGameYear;
+
+    $days = (int) ($interval / $this->secondsPerGameDay);
+    $interval -= $days * $this->secondsPerGameDay;
+
+    $hours = (int) ($interval / $this->secondsPerGameHour);
+    $interval -= $hours * $this->secondsPerGameHour;
+
+    $minutes = (int) ($interval / $this->secondsPerGameMinute);
+    $interval -= $minutes * $this->secondsPerGameMinute;
+
+    $seconds = (int) ($interval / $this->secondsPerGameSecond);
+    $interval -= $seconds * $this->secondsPerGameSecond;
+
+    $ret = clone($this->theBeginning);
+    return $ret->add(
+      $this->interval($years, $days, $hours, $minutes, $seconds)
+    );
+  }
+
+  private function interval(
+    int $years,
+    int $days,
+    int $hours,
+    int $minutes,
+    int $seconds
+  ): \DateInterval {
+    return new \DateInterval(
+      'P'.$years.'Y'.$days.'DT'.$hours.'H'.$minutes.'M'.$seconds.'S'
+    );
+  }
+}
diff --git a/tests/TimeKeeperTest.php b/tests/TimeKeeperTest.php
new file mode 100644
index 0000000..0ff20c0
--- /dev/null
+++ b/tests/TimeKeeperTest.php
@@ -0,0 +1,152 @@
+<?php
+declare(strict_types=1);
+
+namespace LotGD\Core\Tests;
+
+use LotGD\Core\TimeKeeper;
+
+/**
+* @backupGlobals disabled
+* @backupStaticAttributes disabled
+*/
+class TimeKeeperTests extends \PHPUnit_Framework_TestCase {
+  private $gameEpoch;
+  private $gameOffsetSeconds;
+  private $gameDaysPerDay;
+
+  public function setUp() {
+    $this->gameEpoch = new \DateTime('2015-07-27 00:00:00 PDT');;
+    $this->gameOffsetSeconds = 0;
+    $this->gameDaysPerDay = 2;
+  }
+
+  public function testConvertToBasicConversion() {
+    $this->gameDaysPerDay = 1;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $date = new \DateTime('2015-07-27 23:59:59 PDT');
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-01 23:59:59', $converted->format('Y-m-d H:i:s'));
+
+    $date = new \DateTime('2015-07-27 12:00:00 PDT');
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-01 12:00:00', $converted->format('Y-m-d H:i:s'));
+  }
+
+  public function testConvertToRespectsGameDaysPerDayUpperBound() {
+    $date = new \DateTime('2015-07-27 05:59:59 PDT');
+
+    $this->gameDaysPerDay = 4;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-01', $converted->format('Y-m-d'));
+  }
+
+  public function testConvertToRespectsGameDaysPerDayNextDay() {
+    $date = new \DateTime('2015-07-27 06:00:00 PDT');
+
+    $this->gameDaysPerDay = 4;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-02', $converted->format('Y-m-d'));
+  }
+
+  public function testConvertToRespectsGameDaysPerDayNextDayUpperBound() {
+    $date = new \DateTime('2015-07-27 11:59:59 PDT');
+
+    $this->gameDaysPerDay = 4;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-02', $converted->format('Y-m-d'));
+  }
+
+  public function testDetectsNewDay() {
+  }
+
+  public function testConvertToRespectsGameOffset() {
+    $date = new \DateTime('2015-07-27 01:01:15 PDT');
+
+    $this->gameOffsetSeconds = 60*60;
+    $this->gameDaysPerDay = 1;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-01 00:01:15', $converted->format('Y-m-d H:i:s'));
+  }
+
+  public function testConvertToRespectsGameOffsetUpperBound() {
+    $date = new \DateTime('2015-07-28 00:59:59 PDT');
+
+    $this->gameOffsetSeconds = 60*60;
+    $this->gameDaysPerDay = 1;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-01 23:59:59', $converted->format('Y-m-d H:i:s'));
+  }
+
+  public function testConvertToRespectsGameOffsetNextDay() {
+    $date = new \DateTime('2015-07-28 01:00:00 PDT');
+
+    $this->gameOffsetSeconds = 60*60;
+    $this->gameDaysPerDay = 1;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-02', $converted->format('Y-m-d'));
+  }
+
+  public function testConvertToRespectsGameOffsetNextDayUpperBound() {
+    $date = new \DateTime('2015-07-29 00:59:59 PDT');
+
+    $this->gameOffsetSeconds = 60*60;
+    $this->gameDaysPerDay = 1;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertToGameTime($date);
+    $this->assertEquals('0000-01-02', $converted->format('Y-m-d'));
+  }
+
+  public function testConvertFromBasicConversion() {
+    $date = new \DateTime('0000-01-02 00:00:00 UTC');
+
+    $this->gameDaysPerDay = 1;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertFromGameTime($date);
+    $this->assertEquals("2015-07-28", $converted->format('Y-m-d'));
+  }
+
+  public function testConvertFromRespectsGameOffsetNextDay() {
+    $epoch = new \DateTime('2015-07-27 00:00:00 PDT');
+    $date = new \DateTime('0000-01-02 23:59:59 UTC');
+
+    $this->gameEpoch = $epoch;
+    $this->gameOffsetSeconds = 60*60;
+    $this->gameDaysPerDay = 1;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertFromGameTime($date);
+    $this->assertEquals("2015-07-29 00:59:59", $converted->format('Y-m-d H:i:s'));
+  }
+
+  public function testConvertFromRespectsGameDaysPerDayNextDay() {
+    $epoch = new \DateTime('2015-07-27 00:00:00 PDT');
+    $date = new \DateTime('0000-01-02 23:59:59 UTC');
+
+    $this->gameEpoch = $epoch;
+    $this->gameDaysPerDay = 4;
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+
+    $converted = $keeper->convertFromGameTime($date);
+    $this->assertEquals("2015-07-27 11:59:59", $converted->format('Y-m-d H:i:s'));
+  }
+
+  public function testGameTimeSanity() {
+    $keeper = new TimeKeeper($this->gameEpoch, $this->gameOffsetSeconds, $this->gameDaysPerDay);
+    $this->assertNotNull($keeper->gameTime());
+  }
+}