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

README.md

@@ -2,7 +2,7 @@
 
 generic cli timestamp parser and calculator tool
 
-# Usage
+## Usage
 
 ```default
 This is ts, a timestamp tool.
@@ -21,6 +21,28 @@ Usage: ts <time string> [<time string>]
 -e --examples Show examples or supported inputs.
 ```
 
+## Examples
+
+```default
+# diff between to day and yesterday 10 am
+ts today "10am yesterday"
+14h0m0s
+
+# show timestamp from a couple days ago
+ts "3 days ago"
+2025-09-21 13:51:55.054754744 +0200 CEST
+
+# show timestamp of one hour and 45 minutes before (-d is the defaul)
+ts -d now 1h45m
+2025-09-24 12:07:45.072300157 +0200 CEST m=-6299.999710536
+
+# 10 hours from now
+ts now 10h
+2025-09-24 03:53:36.7095512 +0200 CEST m=-35999.999720767
+```
+
+To see a comprehensive list of supported inputs, call `ts -e`.
+
 ## Installation
 
 The tool does not have any dependencies.  Just download the binary for