fix: make engineer tenure timezone independent

components/dashboard-card/time-as-engineer.test.tsx

@@ -0,0 +1,19 @@
+import { render, screen } from '@testing-library/react';
+
+import { TimeAsSoftwareEngineer } from '@/components/dashboard-card/time-as-engineer';
+
+describe('TimeAsSoftwareEngineer', () => {
+  beforeEach(() => {
+    jest.useFakeTimers().setSystemTime(new Date('2026-07-19T12:00:00Z'));
+  });
+
+  afterEach(() => {
+    jest.useRealTimers();
+  });
+
+  it('should render elapsed time as years, months and days', () => {
+    render(<TimeAsSoftwareEngineer />);
+
+    expect(screen.getByText('7y, 11m, 18d')).toBeInTheDocument();
+  });
+});

components/dashboard-card/time-as-engineer.tsx

@@ -0,0 +1,27 @@
+import { useEffect, useState } from 'react';
+
+import { DashboardCard } from '@/components/dashboard-card/dashboard-card';
+import { calendarDurationBetween } from '@/lib/calendar';
+
+const CAREER_START_DATE = new Date('2018-08-01');
+
+export function TimeAsSoftwareEngineer() {
+  const [formattedDuration, setFormattedDuration] = useState<string>();
+
+  // Computed after mount: the page is statically prerendered, so a
+  // day-precise value baked at build time would mismatch on hydration.
+  useEffect(() => {
+    const { years, months, days } = calendarDurationBetween(
+      CAREER_START_DATE,
+      new Date(),
+    );
+    setFormattedDuration(`${years}y, ${months}m, ${days}d`);
+  }, []);
+
+  return (
+    <DashboardCard
+      header="Time as Software Engineer"
+      metric={formattedDuration}
+    />
+  );
+}

lib/calendar.test.ts

@@ -0,0 +1,59 @@
+import { calendarDurationBetween } from '@/lib/calendar';
+
+describe('calendarDurationBetween', () => {
+  const start = new Date('2018-08-01');
+
+  it('should count full years, months and days since start', () => {
+    expect(
+      calendarDurationBetween(start, new Date('2026-07-19T12:00:00Z')),
+    ).toEqual({ years: 7, months: 11, days: 18 });
+  });
+
+  it('should roll over to a new year on the anniversary date', () => {
+    expect(
+      calendarDurationBetween(start, new Date('2026-08-01T00:00:00Z')),
+    ).toEqual({ years: 8, months: 0, days: 0 });
+  });
+
+  it('should stay at the previous year until the anniversary', () => {
+    expect(
+      calendarDurationBetween(start, new Date('2026-07-31T23:59:59Z')),
+    ).toEqual({ years: 7, months: 11, days: 30 });
+  });
+
+  it('should borrow days from the previous month when end day is smaller', () => {
+    expect(
+      calendarDurationBetween(
+        new Date('2018-08-15'),
+        new Date('2026-07-10T00:00:00Z'),
+      ),
+    ).toEqual({ years: 7, months: 10, days: 25 });
+  });
+
+  it('should compute in UTC regardless of local timezone offset', () => {
+    // 23:30 in New York (UTC-4) is already July 20 in UTC
+    expect(
+      calendarDurationBetween(start, new Date('2026-07-19T23:30:00-04:00')),
+    ).toEqual({ years: 7, months: 11, days: 19 });
+  });
+
+  it('should cap month-end anchors to shorter months', () => {
+    expect(
+      calendarDurationBetween(new Date('2020-01-31'), new Date('2020-03-01')),
+    ).toEqual({ years: 0, months: 1, days: 1 });
+  });
+
+  it('should treat a capped february as a full month', () => {
+    expect(
+      calendarDurationBetween(new Date('2020-01-31'), new Date('2021-02-28')),
+    ).toEqual({ years: 1, months: 1, days: 0 });
+  });
+
+  it('should return zeros for the same instant', () => {
+    expect(calendarDurationBetween(start, new Date('2018-08-01'))).toEqual({
+      years: 0,
+      months: 0,
+      days: 0,
+    });
+  });
+});

lib/calendar.ts

@@ -0,0 +1,48 @@
+export interface CalendarDuration {
+  years: number;
+  months: number;
+  days: number;
+}
+
+function daysInUTCMonth(year: number, monthIndex: number): number {
+  // Day 0 of the next month is the last day of monthIndex
+  return new Date(Date.UTC(year, monthIndex + 1, 0)).getUTCDate();
+}
+
+/**
+ * Calendar duration between two instants, computed in UTC so the
+ * result does not depend on the viewer's timezone. Month-end anchors
+ * cap to shorter months (Jan 31 + 1 month = Feb 29), so days are
+ * never negative — the same convention as date-fns.
+ */
+export function calendarDurationBetween(
+  start: Date,
+  end: Date,
+): CalendarDuration {
+  let years = end.getUTCFullYear() - start.getUTCFullYear();
+  let months = end.getUTCMonth() - start.getUTCMonth();
+
+  const anchorDay = Math.min(
+    start.getUTCDate(),
+    daysInUTCMonth(end.getUTCFullYear(), end.getUTCMonth()),
+  );
+  let days = end.getUTCDate() - anchorDay;
+
+  if (days < 0) {
+    months -= 1;
+    const prevMonthDays = daysInUTCMonth(
+      end.getUTCFullYear(),
+      end.getUTCMonth() - 1,
+    );
+    days =
+      end.getUTCDate() +
+      prevMonthDays -
+      Math.min(start.getUTCDate(), prevMonthDays);
+  }
+  if (months < 0) {
+    years -= 1;
+    months += 12;
+  }
+
+  return { years, months, days };
+}

pages/dashboard.tsx

@@ -1,10 +1,10 @@
 import Head from 'next/head';
 
 import { BlogViewsCard } from '@/components/dashboard-card/blog-views';
-import { DaysAsSoftwareEngineer } from '@/components/dashboard-card/days-as-engineer';
 import { GithubFollowers } from '@/components/dashboard-card/github-followers';
 import { GitHubStars } from '@/components/dashboard-card/github-stars';
 import { MonthlyUsers } from '@/components/dashboard-card/monthly-user';
+import { TimeAsSoftwareEngineer } from '@/components/dashboard-card/time-as-engineer';
 
 export default function Dashboard() {
   return (
@@ -53,7 +53,7 @@ export default function Dashboard() {
           Statistics
         </h2>
         <div className="grid gap-4 grid-cols-1 sm:grid-cols-2 my-2 w-full">
-          <DaysAsSoftwareEngineer />
+          <TimeAsSoftwareEngineer />
           <BlogViewsCard />
           <GithubFollowers />
           <GitHubStars />