diff --git a/apps/rpicam_still.cpp b/apps/rpicam_still.cpp
index 5721edc..5ce78ea 100644
--- a/apps/rpicam_still.cpp
+++ b/apps/rpicam_still.cpp
@@ -34,24 +34,28 @@ public:
 
 static std::string generate_filename(StillOptions const *options)
 {
-	char filename[128];
-	std::string folder = options->output; // sometimes "output" is used as a folder name
-	if (!folder.empty() && folder.back() != '/')
-		folder += "/";
+	// treat the output file name as a mask
+	char filename[2048];
 	if (options->datetime)
 	{
 		std::time_t raw_time;
 		std::time(&raw_time);
 		char time_string[32];
 		std::tm *time_info = std::localtime(&raw_time);
-		std::strftime(time_string, sizeof(time_string), "%m%d%H%M%S", time_info);
-		snprintf(filename, sizeof(filename), "%s%s.%s", folder.c_str(), time_string, options->encoding.c_str());
+		std::strftime(time_string, sizeof(time_string), "%Y%m%d%H%M%S", time_info);
+		// expects "%s" in the filename
+		snprintf(filename, sizeof(filename), options->output.c_str(), time_string);
 	}
 	else if (options->timestamp)
-		snprintf(filename, sizeof(filename), "%s%u.%s", folder.c_str(), (unsigned)time(NULL),
-				 options->encoding.c_str());
+	{
+		// expects "%u" in the filename
+		snprintf(filename, sizeof(filename), options->output.c_str(), (unsigned)time(NULL));
+	}
 	else
+	{
+		// expects "%d" in the filename
 		snprintf(filename, sizeof(filename), options->output.c_str(), options->framestart);
+	}
 	filename[sizeof(filename) - 1] = 0;
 	return std::string(filename);
 }
diff --git a/core/still_options.hpp b/core/still_options.hpp
index 1d1fbe7..3338767 100644
--- a/core/still_options.hpp
+++ b/core/still_options.hpp
@@ -27,9 +27,9 @@ struct StillOptions : public Options
 			("framestart", value<uint32_t>(&framestart)->default_value(0),
 			 "Initial frame counter value for timelapse captures")
 			("datetime", value<bool>(&datetime)->default_value(false)->implicit_value(true),
-			 "Use date format for output file names")
+			 "Replace %s with date and time in output file names")
 			("timestamp", value<bool>(&timestamp)->default_value(false)->implicit_value(true),
-			 "Use system timestamps for output file names")
+			 "Replace %u with timestamp in output file names")
 			("restart", value<unsigned int>(&restart)->default_value(0),
 			 "Set JPEG restart interval")
 			("keypress,k", value<bool>(&keypress)->default_value(false)->implicit_value(true),