Move timezone conversion for frontend display to model method

app/models/banner.rb

@@ -12,6 +12,10 @@ def expired?
     expires_at && Time.current > expires_at
   end
 
+  def expires_at_in_time_zone(timezone)
+    expires_at&.in_time_zone(timezone)
+  end
+
   private
 
   def only_one_banner_is_active_per_organization

app/views/banners/_form.html.erb

@@ -28,7 +28,7 @@
     </div>
     <span class="input-style-1">
       <%= form.label :expires_at, "Expires at (optional)" %>
-      <%= form.datetime_field :expires_at, value: banner.expires_at&.in_time_zone(cookies[:browser_time_zone]), required: false %>
+      <%= form.datetime_field :expires_at, value: banner.expires_at_in_time_zone(cookies[:browser_time]), required: false %>
     </span>
     <div class="input-style-1">
       <%= form.label :content %>

spec/models/banner_spec.rb

@@ -42,4 +42,18 @@
       expect(banner).to be_expired
     end
   end
+
+  describe "#expires_at_in_time_zone" do
+    it "can shift time by timezone for equivalent times" do
+      banner = create(:banner, expires_at: "2024-06-13 12:00:00 UTC")
+
+      expires_at_in_pacific_time = banner.expires_at_in_time_zone("America/Los_Angeles")
+      expect(expires_at_in_pacific_time.to_s).to eq("2024-06-13 05:00:00 -0700")
+
+      expires_at_in_eastern_time = banner.expires_at_in_time_zone("America/New_York")
+      expect(expires_at_in_eastern_time.to_s).to eq("2024-06-13 08:00:00 -0400")
+
+      expect(expires_at_in_pacific_time).to eq(expires_at_in_eastern_time)
+    end
+  end
 end