Production Recipes

Real-World Code Examples

Copyable, production-grade TypeScript recipes for common business logic: from identity age verification and subscription billing schedules to hotel stay pricing and database serialization.

Identity & ComplianceBeginner

User Date of Birth & Age Verification

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Validate user-submitted birth dates, verify minimum age requirements (e.g., 18+), and prevent impossible or future birth dates.

date-of-birth.ts

400 font-semibold">import { parseDate, addYears, isBefore, isAfter, todayUtc } 400 font-semibold">from 400 font-semibold">class="text-emerald-400">"daysafe";
400 font-semibold">interface AgeValidationResult {
  valid: boolean;
  error?: string;
  birthDate?: string;
}
400 font-semibold">export 400 font-semibold">function validateBirthDate(rawInput: string, minimumAge = 18): AgeValidationResult {
  400 font-semibold">const birthDate = parseDate(rawInput);
  400 font-semibold">if (!birthDate) {
    400 font-semibold">return { valid: 400 font-semibold">false, error: 400 font-semibold">class="text-emerald-400">"Invalid calendar date. Please enter YYYY-MM-DD." };
  }
  400 font-semibold">const today = todayUtc();
  // 1. Cannot be born in the future
  400 font-semibold">if (isAfter(birthDate, today)) {
    400 font-semibold">return { valid: 400 font-semibold">false, error: 400 font-semibold">class="text-emerald-400">"Date of birth cannot be in the future." };
  }
  // 2. Minimum age threshold (birthDate + minAge <= today)
  400 font-semibold">const requiredBirthday = addYears(birthDate, minimumAge);
  400 font-semibold">if (isAfter(requiredBirthday, today)) {
    400 font-semibold">return { 
      valid: 400 font-semibold">false, 
      error: 400 font-semibold">class="text-emerald-400">`User must be at least ${minimumAge} years old.` 
    };
  }
  400 font-semibold">return { valid: 400 font-semibold">true, birthDate };
}
// Example usage:
console.log(validateBirthDate(400 font-semibold">class="text-emerald-400">"2000-05-15")); 400 font-semibold">class=400 font-semibold">class="text-emerald-400">"text-slate-500 italic">// { valid: 400 font-semibold">true, birthDate: 400 font-semibold">class="text-emerald-400">"2000-05-15" }
console.log(validateBirthDate(400 font-semibold">class="text-emerald-400">"2026-02-30")); 400 font-semibold">class=400 font-semibold">class="text-emerald-400">"text-slate-500 italic">// { valid: 400 font-semibold">false, error: 400 font-semibold">class="text-emerald-400">"Invalid calendar date..." }
console.log(validateBirthDate(400 font-semibold">class="text-emerald-400">"2015-08-10")); 400 font-semibold">class=400 font-semibold">class="text-emerald-400">"text-slate-500 italic">// { valid: 400 font-semibold">false, error: 400 font-semibold">class="text-emerald-400">"User must be at least 18 years old." }

Why DaySafe Matters Here

✓parseDate strictly validates format and calendar reality without silent rollover.
✓addYears handles leap day birthdays (e.g. Feb 29 + 18 years clamps safely to Feb 28).
✓todayUtc ensures verification isn't skewed by client machine clock timezone differences.

Billing & InvoicingIntermediate

Invoice Due Date & Net-30 Payment Terms

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Calculate payment deadlines for invoices (Net 15, Net 30, Net 60) and adjust if due dates fall on weekends.

invoice-due-date.ts

400 font-semibold">import { addDays, isWeekend, getDayOfWeek, formatDate } 400 font-semibold">from 400 font-semibold">class="text-emerald-400">"daysafe";
400 font-semibold">export 400 font-semibold">interface InvoiceTerms {
  issueDate: string;
  netDays: number;
  rollWeekendToMonday?: boolean;
}
400 font-semibold">export 400 font-semibold">function calculateDueDate({
  issueDate,
  netDays,
  rollWeekendToMonday = 400 font-semibold">true,
}: InvoiceTerms) {
  400 font-semibold">let dueDate = addDays(issueDate, netDays);
  400 font-semibold">if (rollWeekendToMonday && isWeekend(dueDate)) {
    400 font-semibold">const dayOfWeek = getDayOfWeek(dueDate);
    // 6 = Saturday (+2 days to Monday), 7 = Sunday (+1 day to Monday)
    400 font-semibold">const daysToAdd = dayOfWeek === 6 ? 2 : 1;
    dueDate = addDays(dueDate, daysToAdd);
  }
  400 font-semibold">const formattedDisplay = formatDate(dueDate, {
    locale: 400 font-semibold">class="text-emerald-400">"en-US",
    style: 400 font-semibold">class="text-emerald-400">"long",
  });
  400 font-semibold">return {
    rawDueDate: dueDate,
    formattedDisplay,
    isRolledOver: isWeekend(addDays(issueDate, netDays)),
  };
}
// Example: Net-30 from Friday Oct 30, 2026 -> falls on Sunday Nov 29 -> rolls to Monday Nov 30
console.log(calculateDueDate({ issueDate: 400 font-semibold">class="text-emerald-400">"2026-10-30", netDays: 30 }));
// { rawDueDate: "2026-11-30", formattedDisplay: "November 30, 2026", isRolledOver: true }

Why DaySafe Matters Here

✓addDays uses pure integer arithmetic, avoiding Daylight Saving Time 23/25 hour bugs.
✓isWeekend checks ISO 8601 day 6 (Saturday) and 7 (Sunday).
✓formatDate produces a timezone-safe customer-facing string without day boundary shifts.

Subscriptions & SaaSIntermediate

Subscription Renewal & Month-End Clamping

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Schedule recurring monthly subscriptions where signups on the 29th, 30th, or 31st safely clamp to month boundaries.

subscription-billing.ts

400 font-semibold">import { addMonths, formatDate, todayUtc } 400 font-semibold">from 400 font-semibold">class="text-emerald-400">"daysafe";
400 font-semibold">export 400 font-semibold">function generateBillingSchedule(startDate: string, durationMonths = 6) {
  400 font-semibold">const schedule: { cycle: number; billingDate: string; formatted: string }[] = [];
  400 font-semibold">for (400 font-semibold">let i = 0; i < durationMonths; i++) {
    400 font-semibold">const billingDate = addMonths(startDate, i);
    schedule.push({
      cycle: i + 1,
      billingDate,
      formatted: formatDate(billingDate, { locale: 400 font-semibold">class="text-emerald-400">"en-US", style: 400 font-semibold">class="text-emerald-400">"medium" }),
    });
  }
  400 font-semibold">return schedule;
}
// User signs up on January 31:
400 font-semibold">const schedule = generateBillingSchedule(400 font-semibold">class="text-emerald-400">"2026-01-31", 4);
console.log(schedule);
/* Output:
  Cycle 1: 400 font-semibold">class="text-emerald-400">"2026-01-31" (Jan 31)
  Cycle 2: 400 font-semibold">class="text-emerald-400">"2026-02-28" (Feb 28 - clamped to month end!)
  Cycle 3: 400 font-semibold">class="text-emerald-400">"2026-03-31" (Mar 31)
  Cycle 4: 400 font-semibold">class="text-emerald-400">"2026-04-30" (Apr 30 - clamped to 30 days)
*/

Why DaySafe Matters Here

✓Standard JS `new Date(2026, 0, 31)` + 1 month rolls over into March 3rd!
✓DaySafe's addMonths clamps to February 28 (or 29 in leap years), preserving monthly cadence.

Hospitality & BookingsBeginner

Hotel Stay & Booking Duration Calculation

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Calculate exact night counts for reservations and validate check-in / check-out ordering.

booking-duration.ts

400 font-semibold">import { daysBetween, isBefore, isSame, addDays } 400 font-semibold">from 400 font-semibold">class="text-emerald-400">"daysafe";
400 font-semibold">export 400 font-semibold">function validateAndPriceBooking(
  checkIn: string,
  checkOut: string,
  ratePerNightUsd: number
) {
  400 font-semibold">if (isSame(checkIn, checkOut)) {
    400 font-semibold">throw 400 font-semibold">new Error(400 font-semibold">class="text-emerald-400">"Check-out date must be at least one day after check-in.");
  }
  400 font-semibold">if (isBefore(checkOut, checkIn)) {
    400 font-semibold">throw 400 font-semibold">new Error(400 font-semibold">class="text-emerald-400">"Check-out date cannot precede check-in date.");
  }
  400 font-semibold">const nights = daysBetween(checkIn, checkOut);
  400 font-semibold">const totalPrice = nights * ratePerNightUsd;
  400 font-semibold">return {
    checkIn,
    checkOut,
    nights,
    totalPrice,
  };
}
// Spring DST transition weekend (e.g. March 7 to March 9):
// Timestamp subtraction with / 86400000 yields 1.958 days on 23-hour DST day!
// DaySafe daysBetween returns exact integer 2 nights:
console.log(validateAndPriceBooking(400 font-semibold">class="text-emerald-400">"2026-03-07", 400 font-semibold">class="text-emerald-400">"2026-03-09", 150));
// { checkIn: "2026-03-07", checkOut: "2026-03-09", nights: 2, totalPrice: 300 }

Why DaySafe Matters Here

✓Eliminates fractional day distortions caused by 23-hour spring-forward or 25-hour fall-back days.
✓Guarantees hotel guests are billed for the exact number of calendar nights.

UI ComponentsAdvanced

Calendar Month View Generation

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Generate complete calendar grids (start of month, end of month, padding days) for month-view UI components.

calendar-grid.ts

400 font-semibold">import { startOfMonth, endOfMonth, getDayOfWeek, addDays, toDateParts } 400 font-semibold">from 400 font-semibold">class="text-emerald-400">"daysafe";
400 font-semibold">export 400 font-semibold">interface CalendarDay {
  date: string;
  dayNumber: number;
  isCurrentMonth: boolean;
  isWeekend: boolean;
}
400 font-semibold">export 400 font-semibold">function buildMonthGrid(referenceDate: string): CalendarDay[] {
  400 font-semibold">const monthStart = startOfMonth(referenceDate);
  400 font-semibold">const monthEnd = endOfMonth(referenceDate);
  400 font-semibold">const { month } = toDateParts(monthStart);
  // ISO day: 1 = Monday ... 7 = Sunday
  400 font-semibold">const startDayOfWeek = getDayOfWeek(monthStart);
  
  // Calculate padding days to start on Monday
  400 font-semibold">const gridStart = addDays(monthStart, -(startDayOfWeek - 1));
  400 font-semibold">const days: CalendarDay[] = [];
  400 font-semibold">let current = gridStart;
  // 6 weeks * 7 days = 42 grid cells
  400 font-semibold">for (400 font-semibold">let i = 0; i < 42; i++) {
    400 font-semibold">const parts = toDateParts(current);
    400 font-semibold">const dayOfWeek = getDayOfWeek(current);
    days.push({
      date: current,
      dayNumber: parts.day,
      isCurrentMonth: parts.month === month,
      isWeekend: dayOfWeek >= 6,
    });
    current = addDays(current, 1);
  }
  400 font-semibold">return days;
}
400 font-semibold">const grid = buildMonthGrid(400 font-semibold">class="text-emerald-400">"2026-11-15");
console.log(400 font-semibold">class="text-emerald-400">`Generated ${grid.length} cells 400 font-semibold">for November 2026`);
console.log(400 font-semibold">class="text-emerald-400">"First grid cell:", grid[0]);

Why DaySafe Matters Here

✓startOfMonth and endOfMonth establish precise boundaries.
✓toDateParts provides clean integers without timezone translation.
✓addDays generates continuous calendar grids seamlessly.

Backend & DatabaseBeginner

PostgreSQL & MySQL DATE Field Validation

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Safely validate and sanitize input for SQL DATE columns without converting through JavaScript Date timestamps.

database-date-fields.ts

400 font-semibold">import { parseDate, DateOnly } 400 font-semibold">from 400 font-semibold">class="text-emerald-400">"daysafe";
400 font-semibold">export 400 font-semibold">class AppointmentModel {
  /**
   * Sanitizes input before executing a database query:
   * INSERT INTO appointments (patient_id, appointment_date) VALUES ($1, $2);
   */
  400 font-semibold">static sanitizeDate(input: unknown): DateOnly {
    400 font-semibold">const validated = parseDate(input);
    400 font-semibold">if (!validated) {
      400 font-semibold">throw 400 font-semibold">new Error(400 font-semibold">class="text-emerald-400">`Invalid SQL DATE value: "${String(input)}". Expected YYYY-MM-DD.`);
    }
    // Returns strictly validated YYYY-MM-DD string, preventing SQL syntax errors
    400 font-semibold">return validated;
  }
}
// Passing to ORMs like Prisma, Drizzle, or raw pg:
400 font-semibold">const safeDate = AppointmentModel.sanitizeDate(400 font-semibold">class="text-emerald-400">"2026-11-02");
console.log(safeDate); 400 font-semibold">class=400 font-semibold">class="text-emerald-400">"text-slate-500 italic">// 400 font-semibold">class="text-emerald-400">"2026-11-02"

Why DaySafe Matters Here

✓SQL DATE columns store calendar dates without time or offset.
✓Passing a standard JS Date to ORMs often converts to UTC midnight timestamp, shifting the day in Western timezones.
✓DaySafe DateOnly strings bind directly to SQL DATE columns with zero ambiguity.