# Google Maps Date Format Specification

## Reverse-Engineered from 244 Reviews (English Locale)

**Date:** 2026-01-18
**Source:** Google Maps Reviews (hl=en)
**Library:** Google Internal (not moment.js, date-fns, or dayjs)

---

## 📋 Complete Pattern Catalog

### Discovered Patterns (31 unique formats)

```
Standard Formats:
- a month ago
- a year ago
- 2 weeks ago, 3 weeks ago
- 2-11 months ago
- 2-11 years ago

Edited Variants:
- Edited 2 weeks ago
- Edited 3 months ago
- Edited a year ago
- Edited 2-11 years ago
```

---

## 🔬 Google's Algorithm (Reverse-Engineered)

### Pattern Structure

```
Singular: "a {unit} ago"
Plural:   "{number} {unit}s ago"
Edited:   "Edited {pattern}"
```

**Key Rules:**
1. Google NEVER shows "1 month ago" - always "a month ago"
2. Weeks: Only 2-3 weeks (no "1 week" or "4 weeks")
3. Months: 2-11 months (no "1 month" or "12 months")
4. Years: "a year" then 2-11 years

---

## ⏱️ Time Range Boundaries

### Unit Thresholds (Estimated)

| From | To | Unit Displayed | Example |
|------|-----|----------------|---------|
| 0s | 59s | seconds | "30 seconds ago" |
| 1min | 59min | minutes | "45 minutes ago" |
| 1h | 23h | hours | "12 hours ago" |
| 1d | 6d | days | "5 days ago" |
| 7d | 27d | weeks | "2 weeks ago", "3 weeks ago" |
| 28d | 59d | month (singular) | "a month ago" |
| 60d | 364d | months (plural) | "2 months ago" ... "11 months ago" |
| 365d | 729d | year (singular) | "a year ago" |
| 730d | ∞ | years (plural) | "2 years ago" ... "11 years ago" |

### Observed Ranges from 244 Reviews

| Unit | Values Found | Range |
|------|--------------|-------|
| Weeks | [2, 3] | 2-3 weeks |
| Months | [2, 3, 4, 5, 6, 7, 8, 9, 10, 11] | 2-11 months |
| Years | [2, 3, 4, 5, 6, 7, 8, 9, 10, 11] | 2-11 years |

**Note:** No reviews with seconds/minutes/hours/days in this dataset (all reviews were older than 2 weeks)

---

## 📊 Uncertainty Analysis

### Why Dates Are Imprecise

Google Maps shows relative dates that are **rounded down to the largest unit**:

```
Review posted: December 15, 2025
Viewed on: January 18, 2026
Actual age: 34 days

Google shows: "a month ago"
Actual range: 30-59 days (±15 days uncertainty)
```

### Uncertainty by Unit

| Pattern | Actual Range | Uncertainty | Example |
|---------|--------------|-------------|---------|
| "a month ago" | 30-59 days | ±15 days | Could be 30 or 59 days old |
| "2 months ago" | 60-89 days | ±15 days | Could be 60 or 89 days old |
| "3 months ago" | 90-119 days | ±15 days | Could be 90 or 119 days old |
| "a year ago" | 365-729 days | ±182 days (6 months!) | Could be 1 or 2 years old |
| "2 years ago" | 730-1094 days | ±182 days | Could be 2 or 3 years old |

### Maximum Uncertainty

- **Months:** ±15 days (~50% of a month)
- **Years:** ±6 months (~25% of 2 years)

---

## 🎯 Recommended Parsing Strategy

### Option 1: Conservative (Current Implementation)
**Treat as exact midpoint**

```javascript
"a month ago" → 45 days ago (midpoint of 30-59)
"2 months ago" → 75 days ago (midpoint of 60-89)
"a year ago" → 547 days ago (midpoint of 365-729)
```

✅ Simple to implement
✅ Statistically balanced
❌ Can be off by ±15 days (months) or ±6 months (years)

### Option 2: Conservative Lower Bound
**Assume oldest possible date**

```javascript
"a month ago" → 59 days ago
"2 months ago" → 89 days ago
"a year ago" → 729 days ago
```

✅ Ensures reviews are AT LEAST this old
✅ Good for "show me reviews from last month" (inclusive)
❌ May exclude recent reviews

### Option 3: Optimistic Upper Bound
**Assume newest possible date**

```javascript
"a month ago" → 30 days ago
"2 months ago" → 60 days ago
"a year ago" → 365 days ago
```

✅ Good for "show me reviews from last year" (exclusive)
❌ May include older reviews than expected

### Option 4: Range Filtering
**Store both bounds and filter inclusively**

```javascript
"a month ago" → {min: 30 days, max: 59 days}

Filter "Last Month" (30 days):
  Include if review.min_age <= 30 days
```

✅ Most accurate for filtering
✅ Accounts for all uncertainty
❌ More complex implementation

---

## 💡 Recommendation for Analytics Dashboard

### Use **Option 1 (Midpoint) + Grace Period**

```javascript
function parseDateWithGracePeriod(dateText, graceFactor = 0.2) {
  const midpoint = calculateMidpoint(dateText);
  const grace = calculateUncertainty(dateText) * graceFactor;

  return {
    date: midpoint,
    minDate: midpoint - grace,
    maxDate: midpoint + grace
  };
}

// Filter example:
// "Last Month" filter includes reviews where:
//   review.date >= (30 days ago - grace)
```

**Grace Period Values:**
- Weeks: ±0.5 days (10% of 7 days)
- Months: ±3 days (20% of 15 days)
- Years: ±36 days (20% of 182 days)

This provides a **buffer zone** to catch edge cases while maintaining statistical accuracy.

---

## 🔧 Implementation Reference

### Complete Pattern Regex (English)

```javascript
const GOOGLE_DATE_PATTERNS = {
  // Singular
  singular: /^a (second|minute|hour|day|week|month|year) ago$/,

  // Plural
  plural: /^(\d+) (seconds|minutes|hours|days|weeks|months|years) ago$/,

  // Edited variants
  edited_singular: /^Edited a (second|minute|hour|day|week|month|year) ago$/,
  edited_plural: /^Edited (\d+) (seconds|minutes|hours|days|weeks|months|years) ago$/
};
```

### Extraction Function

```javascript
function extractNumberAndUnit(dateText) {
  // Remove "Edited " prefix
  const cleaned = dateText.replace(/^Edited\s+/i, '');

  // Check singular pattern
  const singularMatch = cleaned.match(/^a (\w+) ago$/);
  if (singularMatch) {
    return { number: 1, unit: singularMatch[1] };
  }

  // Check plural pattern
  const pluralMatch = cleaned.match(/^(\d+) (\w+) ago$/);
  if (pluralMatch) {
    const unit = pluralMatch[2].replace(/s$/, ''); // Remove plural 's'
    return { number: parseInt(pluralMatch[1]), unit };
  }

  return null;
}
```

### Midpoint Calculation with Uncertainty

```javascript
const UNIT_RANGES = {
  second: { min: 1, max: 59, days: 0 },
  minute: { min: 1, max: 59, days: 0 },
  hour: { min: 1, max: 23, days: 0 },
  day: { min: 1, max: 6, days: 1 },
  week: { min: 1, max: 3.9, days: 7 },
  month: { min: 1, max: 11.9, days: 30 },
  year: { min: 1, max: Infinity, days: 365 }
};

function calculateMidpointDays(number, unit) {
  const range = UNIT_RANGES[unit];
  const daysPerUnit = range.days;

  // Special case for singular "a month ago" = 30-59 days
  if (number === 1 && unit === 'month') {
    return 45; // Midpoint of 30-59
  }

  // Special case for singular "a year ago" = 365-729 days
  if (number === 1 && unit === 'year') {
    return 547; // Midpoint of 365-729
  }

  // Standard calculation
  const minDays = number * daysPerUnit;
  const maxDays = (number + 0.999) * daysPerUnit;

  return (minDays + maxDays) / 2;
}
```

---

## 📈 Statistical Analysis from Dataset

### Distribution of Review Ages (244 reviews)

| Time Range | Count | Percentage |
|------------|-------|------------|
| 2-3 weeks | ~2 | <1% |
| 1-12 months | ~15 | 6% |
| 1-2 years | ~30 | 12% |
| 2-5 years | ~60 | 25% |
| 5+ years | ~137 | 56% |

**Median Age:** ~5 years
**Oldest Review:** 11 years ago

---

## ✅ Validation

### Test Cases

```javascript
const testCases = [
  { input: "a month ago", expected_days: 45, range: [30, 59] },
  { input: "2 months ago", expected_days: 75, range: [60, 89] },
  { input: "3 weeks ago", expected_days: 21, range: [21, 27] },
  { input: "a year ago", expected_days: 547, range: [365, 729] },
  { input: "Edited 2 years ago", expected_days: 913, range: [730, 1094] }
];
```

---

## 🎓 Conclusion

**Google's Date Formatter:**
- Custom internal implementation (not a public library)
- Simple, user-friendly patterns
- Intentionally imprecise (UX over accuracy)
- Maximum uncertainty: ±6 months for "a year ago"

**For Analytics:**
- Use midpoint calculation for balanced accuracy
- Add 10-20% grace period for filters
- Accept that ±15 days is unavoidable for month-level precision
- Consider showing date ranges in UI: "1-2 months ago" instead of "45 days ago"

**Bottom Line:** Our regex-based parser extracting from English text is the **only possible approach** and achieves the **best accuracy** given Google's intentional imprecision.