Make default filename for screenshots use ISO8601 datetime format

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [0.11.0] - Unreleased
+
+### Changed
+
+- The default filename for screenshots uses a datetime format similar to ISO8601, but with reserved characters replaced by underscores https://github.com/Textualize/textual/pull/1518
+
 ## [0.10.0] - 2023-01-19
 
 ### Added

src/textual/app.py

@@ -722,25 +722,29 @@ def save_screenshot(
         self,
         filename: str | None = None,
         path: str = "./",
-        time_format: str = "%Y%m%d %H%M%S %f",
+        time_format: str | None = None,
     ) -> str:
         """Save an SVG screenshot of the current screen.
 
         Args:
             filename: Filename of SVG screenshot, or None to auto-generate
                 a filename with the date and time. Defaults to None.
             path: Path to directory for output. Defaults to current working directory.
-            time_format: Time format to use if filename is None. Defaults to "%Y-%m-%d %X %f".
+            time_format: Date and time format to use if filename is None.
+                Defaults to a format like ISO 8601 with some reserved characters replaced with underscores.
 
         Returns:
             Filename of screenshot.
         """
         if filename is None:
-            svg_filename = (
-                f"{self.title.lower()} {datetime.now().strftime(time_format)}.svg"
-            )
-            for reserved in '<>:"/\\|?*':
-                svg_filename = svg_filename.replace(reserved, "_")
+            if time_format is None:
+                dt = datetime.now().isoformat()
+            else:
+                dt = datetime.now().strftime(time_format)
+            svg_filename_stem = f"{self.title.lower()} {dt}"
+            for reserved in ' <>:"/\\|?*.':
+                svg_filename_stem = svg_filename_stem.replace(reserved, "_")
+                svg_filename = svg_filename_stem + ".svg"
         else:
             svg_filename = filename
         svg_path = os.path.expanduser(os.path.join(path, svg_filename))