diff --git a/devices/common/cicd.yaml b/devices/common/cicd.yaml
index bd51c05..606e4c9 100644
--- a/devices/common/cicd.yaml
+++ b/devices/common/cicd.yaml
@@ -1,5 +1,11 @@
 substitutions:
   friendly_name: ${name}
+esphome:
+  # Externalized firmware-status helpers (date math for "days behind"), kept in
+  # a real C++ header rather than an inline lambda. Path is resolved relative to
+  # the device config dir (devices/).
+  includes:
+    - common/firmware_status.h
 ota:
   - platform: esphome
   - platform: http_request
@@ -63,6 +69,24 @@ text_sensor:
         return {};
       return {latest};
 
+sensor:
+  - platform: template
+    name: Firmware Days Behind
+    id: firmware_days_behind
+    icon: mdi:calendar-clock
+    unit_of_measurement: d
+    accuracy_decimals: 0
+    entity_category: diagnostic
+    # How far out of date this device is. Date math lives in the C++ helper
+    # (common/firmware_status.h); unknown for dev builds or before the first
+    # update check.
+    lambda: |-
+      auto behind = willowbird::days_behind(
+          "${version}", id(firmware_update).update_info.latest_version);
+      if (behind.has_value())
+        return (float) behind.value();
+      return {};
+
 binary_sensor:
   - platform: template
     name: Firmware Up To Date
diff --git a/devices/common/firmware_status.h b/devices/common/firmware_status.h
new file mode 100644
index 0000000..2b58e36
--- /dev/null
+++ b/devices/common/firmware_status.h
@@ -0,0 +1,51 @@
+#pragma once
+//
+// Externalized on-device logic for willowbird firmware status (issue #7/#8).
+//
+// When device logic gets more involved than a one-liner, put it here (a real
+// C++ header with proper tooling) instead of an inline YAML lambda, and call it
+// from a thin lambda. Pulled into a build with:
+//
+//   esphome:
+//     includes:
+//       - common/firmware_status.h
+//
+#include <cstdio>
+#include <optional>
+#include <string>
+
+namespace willowbird {
+
+// Convert the "YYYY.MM.DD" date prefix of a CalVer version into a count of days
+// since the civil epoch (1970-01-01), using Howard Hinnant's days_from_civil.
+// Returns nullopt when the string isn't a YYYY.MM.DD date (e.g. a "dev" build).
+inline std::optional<long> calver_to_days(const std::string &version) {
+  int y = 0, m = 0, d = 0;
+  if (sscanf(version.c_str(), "%d.%d.%d", &y, &m, &d) != 3)
+    return std::nullopt;
+  if (m < 1 || m > 12 || d < 1 || d > 31)
+    return std::nullopt;
+
+  y -= m <= 2;
+  const long era = (y >= 0 ? y : y - 399) / 400;
+  const unsigned yoe = static_cast<unsigned>(y - era * 400);
+  const unsigned doy = (153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1;
+  const unsigned doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+  return era * 146097L + static_cast<long>(doe) - 719468L;
+}
+
+// How many calendar days `current` is behind `latest` (both CalVer versions).
+// Returns nullopt when either version isn't a date (dev build, or no update
+// check has completed yet). Never negative — a device at or ahead of the latest
+// release reports 0.
+inline std::optional<int> days_behind(const std::string &current,
+                                      const std::string &latest) {
+  const auto c = calver_to_days(current);
+  const auto l = calver_to_days(latest);
+  if (!c.has_value() || !l.has_value())
+    return std::nullopt;
+  const long diff = *l - *c;
+  return diff > 0 ? static_cast<int>(diff) : 0;
+}
+
+}  // namespace willowbird