secondsUntilLosingReward util (#5875)

# Motivation

For the periodic confirmation feature, the time until the neuron starts
losing rewards should be displayed. A function to retrieve this value in
seconds has been added here.

# Changes

- New util `secondsUntilLosingReward`.
- Refactor relater utils to use it.

# Tests

- Added.

# Todos

- [ ] Add entry to changelog (if necessary).
not necessary

frontend/src/lib/utils/neuron.utils.ts

@@ -1186,16 +1186,19 @@ const getVotingPowerRefreshedTimestampSeconds = ({
   // to avoid unnecessary notifications.
   fullNeuron?.votingPowerRefreshedTimestampSeconds ?? BigInt(nowInSeconds());
 
+export const secondsUntilLosingRewards = (neuron: NeuronInfo): number => {
+  const rewardLossStart =
+    Number(getVotingPowerRefreshedTimestampSeconds(neuron)) +
+    START_REDUCING_VOTING_POWER_AFTER_SECONDS;
+  return rewardLossStart - nowInSeconds();
+};
+
 export const isNeuronLosingRewards = (neuron: NeuronInfo): boolean =>
-  nowInSeconds() >=
-  getVotingPowerRefreshedTimestampSeconds(neuron) +
-    BigInt(START_REDUCING_VOTING_POWER_AFTER_SECONDS);
+  secondsUntilLosingRewards(neuron) <= 0;
 
 // e.g. "Neuron will start losing rewards in 30 days"
 export const shouldDisplayRewardLossNotification = (
   neuron: NeuronInfo
 ): boolean =>
-  nowInSeconds() >=
-  getVotingPowerRefreshedTimestampSeconds(neuron) +
-    BigInt(START_REDUCING_VOTING_POWER_AFTER_SECONDS) -
-    BigInt(daysToSeconds(NOTIFICATION_PERIOD_BEFORE_REWARD_LOSS_STARTS_DAYS));
+  secondsUntilLosingRewards(neuron) <=
+  daysToSeconds(NOTIFICATION_PERIOD_BEFORE_REWARD_LOSS_STARTS_DAYS);

frontend/src/tests/lib/utils/neuron.utils.spec.ts

@@ -65,6 +65,7 @@ import {
   neuronStakedMaturity,
   neuronVotingPower,
   neuronsVotingPower,
+  secondsUntilLosingRewards,
   shouldDisplayRewardLossNotification,
   sortNeuronsByStake,
   sortNeuronsByVotingPowerRefreshedTimeout,
@@ -3294,6 +3295,34 @@ describe("neuron-utils", () => {
     const losingRewardsPeriod = SECONDS_IN_HALF_YEAR;
     const notificationPeriod = 30 * SECONDS_IN_DAY;
 
+    describe("secondsUntilLosingRewards", () => {
+      it("should return future date when no fullNeuron", () => {
+        expect(
+          secondsUntilLosingRewards({
+            ...mockNeuron,
+            fullNeuron: undefined,
+          })
+        ).toEqual(SECONDS_IN_HALF_YEAR);
+      });
+
+      it("should return seconds until losing rewards", () => {
+        expect(
+          secondsUntilLosingRewards(
+            neuronWithRefreshedTimestamp({
+              votingPowerRefreshedTimestampAgeSecs: 0,
+            })
+          )
+        ).toBe(SECONDS_IN_HALF_YEAR);
+        expect(
+          secondsUntilLosingRewards(
+            neuronWithRefreshedTimestamp({
+              votingPowerRefreshedTimestampAgeSecs: losingRewardsPeriod,
+            })
+          )
+        ).toBe(0);
+      });
+    });
+
     describe("isNeuronLosingRewards", () => {
       it("should return false by default", () => {
         expect(