Don't use ':' in backup directory names

':' is not allowed on FAT/exFAT filesystems, so backups to such
drives failed. Use '%Y-%m-%d_%H-%M-%S' instead; existing backups
with the old format still sort correctly. Document the limitations
of FAT/exFAT in the man page.

man/simple_backup.1

@@ -52,7 +52,15 @@ or to use single or double quotes around them.
 .B \-o, \-\-output DIR
 Specify the directory where the files will be copied. The program will automatically
 create a subdirectory called \(aqsimple_backup\(aq (if it does not already exist) and
-inside this directory the actual backup directory (using the current date and time).
+inside this directory the actual backup directory (using the current date and time, for example
+\(aq2026\-10\-08_17\-34\-47\(aq).
+.RS
+.P
+Backups are best stored on a Linux filesystem (e.g. ext4 or btrfs). FAT and exFAT filesystems
+are supported, but they don\(aqt support hard links, so each backup will be a full copy instead
+of only storing files that changed since the previous one. File ownership and permissions are
+not preserved either.
+.RE
 .TP
 .B \-e, \-\-exclude FILE|DIR|PATTERN [FILE|...]]
 Specify files, directories or patterns to exclude from the backup. Matching files and directories

simple_backup/simple_backup.py

@@ -210,7 +210,9 @@ class Backup:
     # Function to create the actual backup directory
     def define_backup_dir(self) -> None:
         """Define the actual backup dir"""
-        now = datetime.now().strftime('%Y-%m-%d %H:%M:%S')
+        # Avoid ':', which is not allowed on FAT/exFAT filesystems.
+        # Backups created with the old format ('%Y-%m-%d %H:%M:%S') still sort correctly
+        now = datetime.now().strftime('%Y-%m-%d_%H-%M-%S')
         self._output_dir = f'{self.output}/simple_backup/{now}'
 
         if self._remote: