--- a/pkg/common/time.go	Mon May 20 16:28:57 2019 +0200
+++ b/pkg/common/time.go	Mon May 20 16:36:19 2019 +0200
@@ -35,6 +35,8 @@
 	ValueAbove  ValueRangeKind = +1
 )
 
+var utc0 = time.Unix(0, 0)
+
 func InterpolateValueByTime(t1 time.Time, m1 float64, t2 time.Time, m2 float64) func(time.Time) (float64, ValueRangeKind) {
 
 	// f(t1) = m1
@@ -64,7 +66,7 @@
 	}
 
 	a := (m1 - m2) / t1.Sub(t2).Seconds()
-	b := m1 - a*float64(t1.Unix())
+	b := m1 - a*t1.Sub(utc0).Seconds()
 
 	return func(t time.Time) (float64, ValueRangeKind) {
 		switch {
@@ -73,7 +75,7 @@
 		case t.After(max):
 			return 0, ValueAbove
 		default:
-			return a*float64(t.Unix()) + b, ValueInside
+			return a*t.Sub(utc0).Seconds() + b, ValueInside
 		}
 	}
 }
@@ -104,7 +106,7 @@
 	min, max := math.Min(m1, m2), math.Max(m1, m2)
 
 	a := t1.Sub(t2).Seconds() / (m1 - m2)
-	b := float64(t1.Unix()) - m1*a
+	b := t1.Sub(utc0).Seconds() - m1*a
 
 	return func(m float64) (time.Time, ValueRangeKind) {
 		switch {
@@ -113,7 +115,10 @@
 		case m > max:
 			return time.Time{}, ValueAbove
 		default:
-			return time.Unix(int64(math.Ceil(m*a+b)), 0), ValueInside
+			x := m*a + b
+			secs := math.Ceil(x)
+			nsecs := math.Ceil((x - secs) * (999999999 + 1))
+			return time.Unix(int64(secs), int64(nsecs)), ValueInside
 		}
 	}
 }

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/pkg/common/time_test.go	Mon May 20 16:36:19 2019 +0200
@@ -0,0 +1,61 @@
+// This is Free Software under GNU Affero General Public License v >= 3.0
+// without warranty, see README.md and license for details.
+//
+// SPDX-License-Identifier: AGPL-3.0-or-later
+// License-Filename: LICENSES/AGPL-3.0.txt
+//
+// Copyright (C) 2018, 2019 by via donau
+//   – Österreichische Wasserstraßen-Gesellschaft mbH
+// Software engineering by Intevation GmbH
+//
+// Author(s):
+//  * Sascha L. Teichmann <sascha.teichmann@intevation.de>
+
+package common
+
+import (
+	"testing"
+	"time"
+)
+
+func TestInterpolateTimeByValue(t *testing.T) {
+
+	t1 := time.Now().UTC()
+	t2 := t1.Add(time.Hour).UTC()
+
+	f := InterpolateTimeByValue(t1, 10, t2, 20)
+
+	v1, _ := f(10)
+	v2, _ := f(20)
+	v3, _ := f(15)
+
+	t3 := t1.Add(time.Hour / 2)
+
+	d1 := v1.Sub(t1)
+	d2 := v2.Sub(t2)
+	d3 := v3.Sub(t3)
+
+	if d1 < 0 {
+		d1 = -d1
+	}
+
+	if d1 > 100*time.Microsecond {
+		t.Errorf("difference too big t1: %v\n", d1)
+	}
+
+	if d2 < 0 {
+		d2 = -d2
+	}
+
+	if d2 > 100*time.Microsecond {
+		t.Errorf("difference too big t2: %v\n", d2)
+	}
+
+	if d3 < 0 {
+		d3 = -d3
+	}
+
+	if d3 > 100*time.Microsecond {
+		t.Errorf("difference too big t3: %v\n", d3)
+	}
+}