diff --git a/internal/models/models.go b/internal/models/models.go
new file mode 100644
index 0000000..af31c80
--- /dev/null
+++ b/internal/models/models.go
@@ -0,0 +1,81 @@
+// This file is Free Software under the MIT License
+// without warranty, see README.md and LICENSES/MIT.txt for details.
+//
+// SPDX-License-Identifier: MIT
+//
+// SPDX-FileCopyrightText: 2023 German Federal Office for Information Security (BSI) <https://www.bsi.bund.de>
+// Software-Engineering: 2023 Intevation GmbH <https://intevation.de>
+
+// Package models contains helper models used in the tools internally.
+package models
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// TimeRange is a time interval.
+type TimeRange [2]time.Time
+
+// NewTimeInterval creates a new time range.
+// The time values will be sorted.
+func NewTimeInterval(a, b time.Time) TimeRange {
+	if b.Before(a) {
+		a, b = b, a
+	}
+	return TimeRange{a, b}
+}
+
+// guessDate tries to guess an RFC 3339 date time from a given string.
+func guessDate(s string) (time.Time, bool) {
+	for _, layout := range []string{
+		"2006-01-02T15:04:05Z07:00",
+		"2006-01-02T15:04:05",
+		"2006-01-02T15:04",
+		"2006-01-02T15",
+		"2006-01-02",
+		"2006-01",
+		"2006",
+	} {
+		if t, err := time.Parse(s, layout); err == nil {
+			return t, true
+		}
+	}
+	return time.Time{}, false
+}
+
+// UnmarshalText implements [encoding/text.TextUnmarshaler].
+func (tr *TimeRange) UnmarshalText(text []byte) error {
+
+	s := strings.TrimSpace(string(text))
+	// Handle relative case first.
+	if duration, err := time.ParseDuration(s); err == nil {
+		now := time.Now()
+		*tr = NewTimeInterval(now.Add(-duration), now)
+		return nil
+	}
+
+	a, b, found := strings.Cut(s, ",")
+	a, b = strings.TrimSpace(a), strings.TrimSpace(b)
+
+	// Only start date?
+	if !found {
+		start, ok := guessDate(a)
+		if !ok {
+			return fmt.Errorf("%q is not a valid RFC date time", a)
+		}
+		*tr = NewTimeInterval(start, time.Now())
+	}
+	// Real interval
+	start, ok := guessDate(a)
+	if !ok {
+		return fmt.Errorf("%q is not a valid RFC date time", a)
+	}
+	end, ok := guessDate(b)
+	if !ok {
+		return fmt.Errorf("%q is not a valid RFC date time", b)
+	}
+	*tr = NewTimeInterval(start, end)
+	return nil
+}