exit early when no timezone is stored

modules/costs/app/models/time_entry.rb

@@ -123,12 +123,14 @@ def costs_visible_by?(usr)
 
   def start_timestamp
     return nil if start_time.blank?
+    return nil if time_zone.blank?
 
     ActiveSupport::TimeZone[time_zone].local(spent_on.year, spent_on.month, spent_on.day, start_time / 60, start_time % 60)
   end
 
   def end_timestamp
     return nil if end_time.blank?
+    return nil if time_zone.blank?
 
     ActiveSupport::TimeZone[time_zone].local(spent_on.year, spent_on.month, spent_on.day, end_time / 60, end_time % 60)
   end

modules/costs/spec/models/time_entry_spec.rb

@@ -500,6 +500,11 @@ def ensure_membership(project, user, permissions)
       expect(time_entry.start_timestamp).to be_nil
     end
 
+    it "returns nil if timezone is nil" do
+      time_entry.time_zone = nil
+      expect(time_entry.start_timestamp).to be_nil
+    end
+
     it "generates a proper timestamp from the stored information" do
       time_entry.start_time = 14 * 60
       time_entry.spent_on = Date.new(2024, 12, 24)
@@ -515,6 +520,11 @@ def ensure_membership(project, user, permissions)
       expect(time_entry.end_timestamp).to be_nil
     end
 
+    it "returns nil if timezone is nil" do
+      time_entry.time_zone = nil
+      expect(time_entry.end_timestamp).to be_nil
+    end
+
     it "generates a proper timestamp from the stored information" do
       time_entry.end_time = 14 * 60
       time_entry.spent_on = Date.new(2024, 12, 24)