diff --git a/src/emulsion/main.py b/src/emulsion/main.py
index 7c62484..63c5704 100644
--- a/src/emulsion/main.py
+++ b/src/emulsion/main.py
@@ -94,10 +94,16 @@ def prompt_if_missing(args):
 	# Same prompts as config, plus base_date
 	prompt_for_config(args)
 	try:
-		if not args.base_date:
+		while not args.base_date:
 			dflt = datetime.datetime.now().strftime("%Y-%m-%d")
 			resp = input(f"Base date/time for first image [{dflt}]: ").strip()
 			args.base_date = resp if resp else dflt
+			# Validate immediately so we don't crash later
+			try:
+				parse_user_date(args.base_date)
+			except ValueError:
+				print("Invalid format. Please use 'YYYY-MM-DD' or 'YYYY-MM-DD HH:MM:SS'.")
+				args.base_date = None
 	except KeyboardInterrupt:
 		print("\nInterrupted by user. Exiting.")
 		sys.exit(1)
@@ -111,10 +117,10 @@ def parse_user_date(dt_str):
 		return datetime.datetime.strptime(dt_str, "%Y-%m-%d")
 
 
-def build_exiftool_cmd(file_path, author, lab, make, model, film, timestamp):
+def build_exiftool_cmd(file_path, author, lab, make, model, film, timestamp, sidecar_source=None):
 	"""
-	Builds the command list. Does NOT handle dry_run formatting.
-	Always returns a list for subprocess safety.
+	Builds the command list.
+	sidecar_source: If set, we are creating/updating a sidecar FROM this source image.
 	"""
 	current_year = datetime.datetime.now().year
 	cmd = [
@@ -142,7 +148,15 @@ def build_exiftool_cmd(file_path, author, lab, make, model, film, timestamp):
 		cmd.append(f"-UserComment={film}")
 		cmd.append(f"-XMP:Description={film}")
 
-	cmd.append(file_path)
+	if sidecar_source:
+		# Advanced ExifTool usage: read source, write to specific sidecar file
+		# This ensures it works even if the sidecar doesn't exist yet
+		cmd.append(f"-srcfile")
+		cmd.append(file_path)
+		cmd.append(sidecar_source)
+	else:
+		cmd.append(file_path)
+
 	return cmd
 
 
@@ -166,6 +180,8 @@ def run_exiftool(cmd, dry_run=False):
 		return True, "Updated"
 	except subprocess.CalledProcessError as e:
 		return False, f"Error: {e}"
+	except FileNotFoundError:
+		return False, "Error: 'exiftool' not found. Please install it."
 
 
 def create_config_file(args):
@@ -201,9 +217,6 @@ def main():
 		if args.workers is None:
 			args.workers = os.cpu_count() or 1
 
-		# LOGIC SIMPLIFICATION 1:
-		# Instead of writing a separate sequential loop for dry-runs,
-		# we just force workers=1 here. The executor handles the rest.
 		if args.dry_run:
 			print("Dry run detected: Forcing sequential processing.")
 			args.workers = 1
@@ -243,8 +256,12 @@ def main():
 				ts_dt = base_dt + datetime.timedelta(seconds=i * time_increment)
 				timestamp_str = ts_dt.strftime("%Y:%m:%d %H:%M:%S")
 
+				sidecar_source = None
 				if args.sidecar:
 					target_file_path = f"{f}.xmp"
+					# If sidecar doesn't exist, we must tell ExifTool to create it from the source image
+					if not os.path.exists(target_file_path):
+						sidecar_source = f
 				else:
 					target_file_path = f
 
@@ -255,14 +272,14 @@ def main():
 					make=args.make,
 					model=args.model,
 					film=args.film,
-					timestamp=timestamp_str
+					timestamp=timestamp_str,
+					sidecar_source=sidecar_source
 				)
 
 				tasks.append((cmd, f, timestamp_str))
 
 			with ThreadPoolExecutor(max_workers=args.workers) as executor:
 				# Submit all tasks
-				# Note: We pass args.dry_run into the function here
 				futures = {
 					executor.submit(run_exiftool, cmd, args.dry_run): (f, ts)
 					for cmd, f, ts in tasks
@@ -274,8 +291,6 @@ def main():
 					success, msg = future.result()
 
 					if args.dry_run:
-						# For dry run, we PRINT the command so the user can copy it
-						# bar.text() is transient, print() persists in terminal
 						print(msg)
 					elif not success:
 						bar.text(f"Failed {original_file}: {msg}")