tools/time: introduce TimeAtOrIn to calculate absolute time from optional relative

tools/time_tools.go

@@ -0,0 +1,13 @@
+package tools
+
+import "time"
+
+// TimeAtOrIn returns either "at", or the "in" duration added to the current
+// time. TimeAtOrIn prefers to add a duration rather than return the "at"
+// parameter.
+func TimeAtOrIn(at time.Time, in time.Duration) time.Time {
+	if in == 0 {
+		return at
+	}
+	return time.Now().Add(in)
+}