time: - operator overloading (#7259)

vlib/time/time_test.v

@@ -164,6 +164,20 @@ fn test_str() {
 	assert '1980-07-11 21:23:42' == time_to_test.str()
 }
 
+fn test_subtract() {
+	d_seconds := 3
+	d_microseconds := 13
+	duration := d_seconds * time.second + d_microseconds * time.microsecond
+	t1 := time_to_test
+	t2 := time.unix2(int(t1.unix) + d_seconds, t1.microsecond + d_microseconds)
+	d1 := t2 - t1
+	d2 := t1 - t2
+	assert d1 > 0
+	assert d1 == duration
+	assert d2 < 0
+	assert d2 == -duration
+}
+
 // not optimal test but will find obvious bugs
 fn test_now() {
 	now := time.now()