date-threshold-arithmetic

Date Threshold Arithmetic

The Problem

Date comparisons at thresholds fail due to fractional days:

const daysBetween = (Date.now() - someDate.getTime()) / (1000 * 60 * 60 * 24);

// someDate is "exactly 7 days ago"
// daysBetween = 7.000012 (due to milliseconds)
// if (daysBetween > 7) { ... } // TRUE — unexpected!

The Solution

Always Math.floor() before threshold comparison.

function daysSince(date) {
  const ms = Date.now() - new Date(date).getTime();
  return Math.floor(ms / (1000 * 60 * 60 * 24));
}

// Now "exactly 7 days" = 7, not 7.000012
if (daysSince(lastCheck) > 7) {
  // Truly more than 7 full days
}

Common Traps

Code	Bug
days > 7	Triggers on 7.0001
days >= 7	Doesn't trigger on 6.9999
days === 7	Never true for fractional values

Safe Patterns

// Days since (full days only)
const fullDays = Math.floor((now - then) / MS_PER_DAY);

// Is it past the threshold?
if (fullDays > threshold) { ... }

// Is it on or past the threshold?
if (fullDays >= threshold) { ... }

// Is it exactly N days?
if (fullDays === threshold) { ... }

For Cron-Style Checks

// "Run if last run was more than 24 hours ago"
const lastRun = new Date(stored.lastRunTime);
const hoursSince = (Date.now() - lastRun.getTime()) / (1000 * 60 * 60);

// Use Math.floor for "full hours"
if (Math.floor(hoursSince) >= 24) {
  runTask();
}

Verification

// Test edge case
const exactlySevenDaysAgo = new Date(Date.now() - 7 * 24 * 60 * 60 * 1000);
console.log(daysSince(exactlySevenDaysAgo)); // Should be 7, not 7.xxx

When to Apply

• Cache expiration checks
• "Last N days" filters
• Scheduled task triggers
• Any date-based threshold

Tags

quality dates javascript bugs