diff --git a/pyproject.toml b/pyproject.toml
index 3a5256f..9e1fd81 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "qsv"
-version = "0.3.5"
+version = "0.3.6"
 description = "A tool that provides elastic and rapid filtering for efficient analysis of huge CSV files, such as eventlogs."
 readme = "README.md"
 authors = [
diff --git a/src/qsv/controllers/DataFrameController.py b/src/qsv/controllers/DataFrameController.py
index deae1ec..af7732e 100644
--- a/src/qsv/controllers/DataFrameController.py
+++ b/src/qsv/controllers/DataFrameController.py
@@ -183,11 +183,14 @@ def changetz(
         logger.debug(f"change {colname} timezone {timezone_from} to {timezone_to}.")
         self.__check_exists_colnames([colname])
 
-        if datetime_format:
-            self.df = self.df.with_columns(pl.col(colname).str.to_datetime(datetime_format))
-        else:
-            self.df = self.df.with_columns(pl.col(colname).str.to_datetime())
-
+        # convert string to datetime
+        if self.df.select(colname).collect_schema().dtypes()[0] != pl.Datetime:
+            if datetime_format:
+                self.df = self.df.with_columns(pl.col(colname).str.to_datetime(datetime_format))
+            else:
+                self.df = self.df.with_columns(pl.col(colname).str.to_datetime())
+
+        # setup and change timezone
         self.df = self.df.with_columns(pl.col(colname).dt.replace_time_zone(timezone_from))
         self.df = self.df.with_columns(pl.col(colname).dt.convert_time_zone(timezone_to))
         return self
diff --git a/uv.lock b/uv.lock
index 4b6201e..3a214e6 100644
--- a/uv.lock
+++ b/uv.lock
@@ -215,7 +215,7 @@ wheels = [
 
 [[package]]
 name = "qsv"
-version = "0.3.2"
+version = "0.3.6"
 source = { editable = "." }
 dependencies = [
     { name = "fire" },
@@ -224,7 +224,7 @@ dependencies = [
     { name = "rich" },
 ]
 
-[package.dependency-groups]
+[package.dev-dependencies]
 dev = [
     { name = "pdoc" },
     { name = "pytest" },
@@ -238,7 +238,7 @@ requires-dist = [
     { name = "rich", specifier = ">=13.9.3" },
 ]
 
-[package.metadata.dependency-groups]
+[package.metadata.requires-dev]
 dev = [
     { name = "pdoc", specifier = ">=15.0.0" },
     { name = "pytest", specifier = ">=8.3.3" },