Relative Time

const timeAgo = (date) => {
  const seconds = Math.floor((Date.now() - new Date(date)) / 1000);
  const intervals = [
    { label: "year",   seconds: 31536000 },
    { label: "month",  seconds: 2592000  },
    { label: "week",   seconds: 604800   },
    { label: "day",    seconds: 86400    },
    { label: "hour",   seconds: 3600     },
    { label: "minute", seconds: 60       },
    { label: "second", seconds: 1        },
  ];
  for (const { label, seconds: s } of intervals) {
    const count = Math.floor(seconds / s);
    if (count >= 1) return `${count} ${label}${count > 1 ? "s" : ""} ago`;
  }
  return "just now";
};

timeAgo(new Date(Date.now() - 30000));           // "30 seconds ago"
timeAgo(new Date(Date.now() - 90000));           // "1 minute ago"
timeAgo(new Date(Date.now() - 7200000));         // "2 hours ago"
timeAgo(new Date(Date.now() - 86400000 * 3));   // "3 days ago"
timeAgo(new Date(Date.now() - 86400000 * 400)); // "1 year ago"

const timeFrom = (date) => {
  const seconds = Math.floor((new Date(date) - Date.now()) / 1000);
  if (seconds < 0) return timeAgo(date); // already passed
  const intervals = [
    { label: "year",   seconds: 31536000 },
    { label: "month",  seconds: 2592000  },
    { label: "week",   seconds: 604800   },
    { label: "day",    seconds: 86400    },
    { label: "hour",   seconds: 3600     },
    { label: "minute", seconds: 60       },
    { label: "second", seconds: 1        },
  ];
  for (const { label, seconds: s } of intervals) {
    const count = Math.floor(seconds / s);
    if (count >= 1) return `in ${count} ${label}${count > 1 ? "s" : ""}`;
  }
  return "just now";
};

timeFrom(new Date(Date.now() + 3600000));      // "in 1 hour"
timeFrom(new Date(Date.now() + 86400000 * 5)); // "in 5 days"
timeFrom(new Date(Date.now() + 60000));        // "in 1 minute"

const rtf = new Intl.RelativeTimeFormat("en", { numeric: "auto" });

rtf.format(-1, "day");    // "yesterday"
rtf.format(1, "day");     // "tomorrow"
rtf.format(-3, "hour");   // "3 hours ago"
rtf.format(2, "week");    // "in 2 weeks"
rtf.format(-1, "month");  // "last month"
rtf.format(1, "year");    // "next year"

// Helper that picks the right unit automatically
const relativeTime = (date) => {
  const rtf = new Intl.RelativeTimeFormat("en", { numeric: "auto" });
  const diff = (new Date(date) - Date.now()) / 1000;
  const units = [
    { unit: "year",   threshold: 31536000 },
    { unit: "month",  threshold: 2592000  },
    { unit: "week",   threshold: 604800   },
    { unit: "day",    threshold: 86400    },
    { unit: "hour",   threshold: 3600     },
    { unit: "minute", threshold: 60       },
    { unit: "second", threshold: 1        },
  ];
  for (const { unit, threshold } of units) {
    if (Math.abs(diff) >= threshold) {
      return rtf.format(Math.round(diff / threshold), unit);
    }
  }
  return "just now";
};

relativeTime(new Date(Date.now() - 90000));          // "2 minutes ago"
relativeTime(new Date(Date.now() + 86400000));       // "tomorrow"
relativeTime(new Date(Date.now() - 86400000 * 2));  // "2 days ago"

const formatDuration = (totalSeconds) => {
  const days    = Math.floor(totalSeconds / 86400);
  const hours   = Math.floor((totalSeconds % 86400) / 3600);
  const minutes = Math.floor((totalSeconds % 3600) / 60);
  const seconds = totalSeconds % 60;

  const parts = [];
  if (days)    parts.push(`${days}d`);
  if (hours)   parts.push(`${hours}h`);
  if (minutes) parts.push(`${minutes}m`);
  if (seconds || parts.length === 0) parts.push(`${seconds}s`);
  return parts.join(" ");
};

formatDuration(45);      // "45s"
formatDuration(90);      // "1m 30s"
formatDuration(3661);    // "1h 1m 1s"
formatDuration(90061);   // "1d 1h 1m 1s"

const formatDurationHuman = (ms) => {
  const seconds = Math.floor(ms / 1000);
  const minutes = Math.floor(seconds / 60);
  const hours   = Math.floor(minutes / 60);
  const days    = Math.floor(hours / 24);

  if (days > 0)    return `${days} day${days > 1 ? "s" : ""}`;
  if (hours > 0)   return `${hours} hour${hours > 1 ? "s" : ""}`;
  if (minutes > 0) return `${minutes} minute${minutes > 1 ? "s" : ""}`;
  return `${seconds} second${seconds !== 1 ? "s" : ""}`;
};

formatDurationHuman(1000);      // "1 second"
formatDurationHuman(90000);     // "1 minute"
formatDurationHuman(7200000);   // "2 hours"
formatDurationHuman(172800000); // "2 days"

const countdown = (targetDate, onTick, onDone) => {
  const tick = () => {
    const remaining = Math.max(0, Math.floor((new Date(targetDate) - Date.now()) / 1000));
    const days    = Math.floor(remaining / 86400);
    const hours   = Math.floor((remaining % 86400) / 3600);
    const minutes = Math.floor((remaining % 3600) / 60);
    const seconds = remaining % 60;

    onTick({ days, hours, minutes, seconds, remaining });

    if (remaining <= 0) {
      onDone?.();
    } else {
      setTimeout(tick, 1000);
    }
  };
  tick();
};

// Usage
countdown(
  "2025-12-31T23:59:59",
  ({ days, hours, minutes, seconds }) => {
    console.log(`${days}d ${hours}h ${minutes}m ${seconds}s`);
  },
  () => console.log("Happy New Year!")
);

const isSameDay = (a, b) => {
  const da = new Date(a), db = new Date(b);
  return da.getFullYear() === db.getFullYear() &&
         da.getMonth()    === db.getMonth() &&
         da.getDate()     === db.getDate();
};

const isToday     = (date) => isSameDay(date, new Date());
const isYesterday = (date) => isSameDay(date, new Date(Date.now() - 86400000));
const isTomorrow  = (date) => isSameDay(date, new Date(Date.now() + 86400000));

isToday(new Date());                       // true
isYesterday(new Date(Date.now() - 86400000)); // true
isTomorrow(new Date(Date.now() + 86400000));  // true

const getDaysBetween = (a, b) =>
  Math.abs(Math.floor((new Date(b) - new Date(a)) / 86400000));

getDaysBetween("2024-01-01", "2024-12-31"); // 365
getDaysBetween(new Date(), "2025-01-01");   // days until new year