cleanup/small refactor + first tests

Backend/helper/time.go

@@ -0,0 +1,16 @@
+package helper
+
+import (
+	"time"
+)
+
+func GetMonday(ts time.Time) time.Time {
+	if ts.Weekday() != time.Monday {
+		if ts.Weekday() == time.Sunday {
+			return ts.AddDate(0, 0, -6)
+		} else {
+			return ts.AddDate(0, 0, -int(ts.Weekday()-1))
+		}
+	}
+	return ts
+}