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

cmd/config.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 cmd
 
 import (
@@ -14,12 +31,12 @@ import (
 )
 
 const (
-	VERSIONstring        = "0.0.1"
+	VERSIONstring        = "0.0.2"
 	Usage         string = `This is ts, a timestamp tool.
 
 Usage: ts <time string> [<time string>]
 -d --diff     Calculate difference between two timestamps (default)
--a --add      Add two timestamps (second parameter must be a time)
+-a --add      Add two timestamps (second parameter must be a duration)
 -f --format   For diffs: duration, hour, min, sec, msec
               For timestamps: datetime, rfc3339, date, time, unix, string
               string is a strftime(1) format string. datetime is
@@ -54,7 +71,11 @@ noon                   Yesterday at 10:15am                 Mon, 02 Jan 2006 15:
 1:05pm                 Next dec 22nd at 3pm                 3 days ago                            1999AD
 10:25:10am             Next December 25th at 7:30am UTC-7   Three days ago                        1999 AD
 1:05:10pm              Next December 23rd AT 5:25 PM        1 day from now                        2008CE
-10:25                  Last December 23rd AT 5:25 PM        1 week ago                            2008 CE`
+10:25                  Last December 23rd AT 5:25 PM        1 week ago                            2008 CE
+
+Example durations for second parameter:
+2d1h30m  2 days, one and a half hour
+30m      30 minutes`
 	ModeDiff int = iota
 	ModeAdd
 )
@@ -110,6 +131,15 @@ func InitConfig(output io.Writer) (*Config, error) {
 		return nil, fmt.Errorf("error unmarshalling: %w", err)
 	}
 
+	// want examples?
+	if conf.Examples {
+		_, err := fmt.Fprintln(output, Examples)
+		if err != nil {
+			Die(err)
+		}
+		os.Exit(0)
+	}
+
 	// args are timestamps
 	if len(flagset.Args()) == 0 {
 		return nil, errors.New("no timestamp argument[s] specified.\n" + Usage)