allow datetime string as --timestamp parameter

internal/cli/transfer.go

@@ -16,8 +16,8 @@ Scotty. If not, see <https://www.gnu.org/licenses/>.
 package cli
 
 import (
+	"errors"
 	"fmt"
-	"math"
 	"strconv"
 	"sync"
 	"time"
@@ -155,13 +155,29 @@ func (c *TransferCmd[E, I, R]) Transfer(exp backends.ExportProcessor[R], imp bac
 }
 
 func (c *TransferCmd[E, I, R]) timestamp() (time.Time, error) {
-	flagValue, err := c.cmd.Flags().GetInt64("timestamp")
-	if err == nil && flagValue > math.MinInt64 {
-		return time.Unix(flagValue, 0), nil
-	} else {
+	flagValue, err := c.cmd.Flags().GetString("timestamp")
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	// No timestamp given, read from database
+	if flagValue == "" {
 		timestamp, err := c.db.GetImportTimestamp(c.sourceName, c.targetName, c.entity)
 		return timestamp, err
 	}
+
+	// Try using given value as a Unix timestamp
+	if timestamp, err := strconv.ParseInt(flagValue, 10, 64); err == nil {
+		return time.Unix(timestamp, 0), nil
+	}
+
+	// Try to parse datetime string
+	for _, format := range []string{time.DateTime, time.RFC3339} {
+		if t, err := time.Parse(format, flagValue); err == nil {
+			return t, nil
+		}
+	}
+	return time.Time{}, errors.New(i18n.Tr("invalid timestamp string \"%v\"", flagValue))
 }
 
 func (c *TransferCmd[E, I, R]) updateTimestamp(result models.ImportResult, oldTimestamp time.Time) error {