diff --git a/app/controllers/challenge.py b/app/controllers/challenge.py
index c67e35d..00edf82 100644
--- a/app/controllers/challenge.py
+++ b/app/controllers/challenge.py
@@ -131,14 +131,15 @@ def main(challenge_id_untrusted):
         raise ValueError
     
     challenge_data = common_helpers.get_challenge(challenge_id)
+    # If challenge not found, quietly redirect to homepage
     if not challenge_data:
-#        flash("Tätä haastetta ei löydy.", "info")
         return {"redirect": True, "url": "/"}
     
-    # If challenge is still a draft, redirect to homepage
+    # If challenge is still a draft, quietly redirect to homepage
     if challenge_data["status"] == "draft":
-#        flash("Tämä haaste ei ole juuri nyt avoinna.", "info")
         return {"redirect": True, "url": "/"}
+    
+    # TODO: If challenge is not open anymore
 
     # Participation data
     # Logged in user
diff --git a/app/controllers/participation.py b/app/controllers/participation.py
index 98f7f73..41be3b7 100644
--- a/app/controllers/participation.py
+++ b/app/controllers/participation.py
@@ -248,6 +248,10 @@ def validate_participation_data(form_data):
     else:
         form_data["place"] = ""
 
+    # 0) Add trashed if not present
+    if "trashed" not in form_data:
+        form_data["trashed"] = False
+
     # Handle taxon data
     # 1) Extract taxon data from form data
     taxa_data = taxa_to_dict(form_data)
@@ -261,13 +265,23 @@ def validate_participation_data(form_data):
         if not common_helpers.is_yyyy_mm_dd(value):
             del taxa_data[key]
 
-    # 4) Add trashed if not present
-    if "trashed" not in form_data:
-        form_data["trashed"] = False
-
-    # 5) Calculate number of species
+    # 4) Calculate number of species
     form_data["taxa_count"] = len(taxa_data)
 
+    # 5) Compare to date_fields_count, to see if all fields were received from the browser
+    #    Do this only if user is not trashing / has not trashed the participation
+    if form_data["trashed"] == True or form_data["trashed"] == 1 or form_data["trashed"] == "1":
+        print("Trashed, skipping date_fields_count check.")
+    else:
+        if "date_fields_count" in form_data:
+#            print("Taxa count: ", form_data["taxa_count"])
+#            print("Date field count: ", form_data["date_fields_count"])
+
+            if form_data["taxa_count"] != int(form_data["date_fields_count"]):
+                raise Exception(f"Received incomplete data from browser (expected { form_data['date_fields_count'] } taxa, received { form_data['taxa_count'] }).")
+        else:
+            raise Exception("Received incomplete data from browser (date_fields_count missing).")
+
     # 6) Convert to JSON string (for database JSON field) where dates are YYYY-MM-DD
     # Note: form_data still has the original taxon data
     form_data["taxa_json"] = json.dumps(taxa_data)
@@ -383,10 +397,6 @@ def main(challenge_id_untrusted, participation_id_untrusted, form_data = None):
         # Todo: Maybe instead remove only empty taxon fields here?
         form_data = {key: value for key, value in form_data.items(multi=True) if value}
 
-        # First check that form is fully received by checking that field form_completed has value "ready"
-        if "form_completed" not in form_data or form_data["form_completed"] != "ready":
-            raise Exception("Received incomplete data from browser.")
-
         errors, form_data = validate_participation_data(form_data)
 
         # Case C1: Errors found. Show the form again with error messages.
diff --git a/app/templates/form_challenge100.html b/app/templates/form_challenge100.html
index bff9e56..89906fc 100644
--- a/app/templates/form_challenge100.html
+++ b/app/templates/form_challenge100.html
@@ -152,6 +152,28 @@
         .then(data => speciesData = data);
 
     document.addEventListener('DOMContentLoaded', function() {
+
+        // Count filled in date field before submission 
+        // Todo: Change form name to participation. Side-effects?
+        document.getElementById('challenge').addEventListener('submit', function(event) {
+            
+            // Select all date inputs with the class 'species_date'
+            var dateFields = document.querySelectorAll('#challenge input[type="date"]');
+            var filledCount = 0;
+
+            // Loop through each date field to check if it's filled
+            dateFields.forEach(function(field) {
+                if (field.value) {  // If the field is not empty (i.e., has a value)
+                    filledCount++;
+                }
+            });
+
+            // Set the count in the hidden input field
+            document.getElementById('date_fields_count').value = filledCount;
+            console.log("Number of filled date fields: " + filledCount);
+        });
+
+        // Autocmplete logic
         const input = document.getElementById('autocomplete-input');
         const resultsContainer = document.getElementById('autocomplete-results');
         
@@ -215,7 +237,7 @@
             }, 300); // Wait 300 ms before processing the input
         });
     });
-
+  
     </script>
 
 {% endblock %}
@@ -276,6 +298,9 @@ <h1>Osallistuminen: {{ html['challenge']['title'] }}</h1>
             >
         </p>
 
+        <!-- Hidden input field to store the count of filled date fields -->
+        <input type="hidden" id="date_fields_count" name="date_fields_count">
+
         <p id="submit_container">
             <button type="submit" id="submit_button">Tallenna osallistuminen</button>
         </p>
@@ -316,9 +341,6 @@ <h3 id="taxon_count_heading">{{ html['data_fields']['taxa_count'] }} lajia</h3>
         {{ html['taxa']|safe }}
         </fieldset>
 
-        <!-- Hidden form field -->
-        <input type="hidden" name="form_completed" value="ready">
-
     </form>
 
 {% endblock %}
diff --git a/tests-playwright/test_login.py b/tests-playwright/test_login.py
index 3aeb971..fdd7690 100644
--- a/tests-playwright/test_login.py
+++ b/tests-playwright/test_login.py
@@ -157,6 +157,7 @@ def test_add_edit_participation(browser):
     assert page.input_value("#MX_71663") == ""
 
     # Trash the participation
+    page.fill("#MX_71822", "2024-01-01") # Add one more taxon to test trashing with changed taxon count
     page.click("#trash_button")
     page.click("#confirm_button")