Use interfaces, add unit tests, add duration diff support (#3)

* use interface for Duration and Time printing, fix diff ts parsing
* add unit tests and gpl
* added tests, add support for duration diff+add
* bump version
* catch setenv errors
* fix ci job name

main.go

@@ -1,3 +1,20 @@
+/*
+Copyright © 2025 Thomas von Dein
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.
+*/
+
 package main
 
 import (