diff --git a/bundles/org.openhab.core/src/main/java/org/eclipse/smarthome/core/library/types/DateTimeType.java b/bundles/org.openhab.core/src/main/java/org/eclipse/smarthome/core/library/types/DateTimeType.java
index b96189dde3a..9a0fd58a036 100644
--- a/bundles/org.openhab.core/src/main/java/org/eclipse/smarthome/core/library/types/DateTimeType.java
+++ b/bundles/org.openhab.core/src/main/java/org/eclipse/smarthome/core/library/types/DateTimeType.java
@@ -104,10 +104,11 @@ public DateTimeType(String zonedValue) {
                     date = parse("1970-01-01T" + zonedValue);
                 } catch (DateTimeParseException timeOnlyException) {
                     try {
-                        Long epoch = Long.valueOf(zonedValue);
+                        long epoch = Double.valueOf(zonedValue).longValue();
+                        int length = (int) (Math.log10(epoch >= 0 ? epoch : epoch * -1) + 1);
                         Instant i;
                         // Assume that below 12 digits we're in seconds
-                        if (zonedValue.length() < 12) {
+                        if (length < 12) {
                             i = Instant.ofEpochSecond(epoch);
                         } else {
                             i = Instant.ofEpochMilli(epoch);
diff --git a/bundles/org.openhab.core/src/test/java/org/eclipse/smarthome/core/library/types/DecimalTypeTest.java b/bundles/org.openhab.core/src/test/java/org/eclipse/smarthome/core/library/types/DecimalTypeTest.java
index 607536dd5af..0e3b75410d3 100644
--- a/bundles/org.openhab.core/src/test/java/org/eclipse/smarthome/core/library/types/DecimalTypeTest.java
+++ b/bundles/org.openhab.core/src/test/java/org/eclipse/smarthome/core/library/types/DecimalTypeTest.java
@@ -121,6 +121,12 @@ public void testConversionToHSBType() {
     public void testConversionToDateTimeType() {
         assertEquals(new DateTimeType("2014-03-30T10:58:47+0000"),
                 new DecimalType("1396177127").as(DateTimeType.class));
+        assertEquals(new DateTimeType("1969-12-31T23:59:59+0000"), new DecimalType("-1").as(DateTimeType.class));
+        assertEquals(new DateTimeType("1970-01-01T00:00:00+0000"), DecimalType.ZERO.as(DateTimeType.class));
+        assertEquals(new DateTimeType("1970-01-01T00:00:01+0000"), new DecimalType("1").as(DateTimeType.class));
+        assertEquals(new DateTimeType("1970-01-01T00:00:01+0000"), new DecimalType("1.0").as(DateTimeType.class));
+        assertEquals(new DateTimeType("1970-01-01T00:00:01+0000"), new DecimalType("1.5").as(DateTimeType.class));
+        assertEquals(new DateTimeType("1969-12-31T23:59:59+0000"), new DecimalType("-1.0").as(DateTimeType.class));
     }
 
     @Test