Fix date calculation bug and add test

prodos/time.go

@@ -45,7 +45,7 @@ func DateTimeFromProDOS(buffer []byte) time.Time {
 	}
 
 	month := int(buffer[0]>>5 + buffer[1]&1)
-	day := int(buffer[0] & 23)
+	day := int(buffer[0] & 31)
 	hour := int(buffer[3])
 	minute := int(buffer[2])
 

prodos/time_test.go

@@ -0,0 +1,15 @@
+package prodos
+
+import (
+	"testing"
+	"time"
+)
+
+func TestDateTimeToAndFromProDOS(t *testing.T) {
+	now := time.Now().Round(time.Minute)
+
+	got := DateTimeFromProDOS(DateTimeToProDOS(now))
+	if got != now {
+		t.Errorf("DateTimeFromProDOS(DateTimeToProDOS(now)) = %s; want %s", got.String(), now.String())
+	}
+}