getDayIndexFromDateString — GTM Variable Template for Date

VARIABLES › DATE

getDayIndexFromDateString EXTENDED Date

Direct (.tpl) Apply (.tpl) Add to GTM

Returns the ISO 8601 day of the week (1 = Monday, 7 = Sunday) from a date string.

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When to Use This

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Examples

Saturday returns 6▶ Try

INPUT

Date String: 2024-11-02

OUTPUT

6

Monday returns 1▶ Try

INPUT

Date String: 2024-12-23

OUTPUT

1

Invalid format returns undefined▶ Try

INPUT

Date String: 25/12/2024

OUTPUT

undefined

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Live Sandbox

This is what you'll see when you open this variable in Google Tag Manager. Hover the icons for details.

getDayIndexFromDateString

Read-Only Try Live

Date String

💾 A date string in YYYY-MM-DD format.

Supported formats:
  ✓ String

Input Setup

Input Function (optional)

⚙️ Optional pre-processing function applied to the input before internal logic (e.g., convert object to string, normalize case). Internal transformations such as case handling will still apply afterward.

Result Handling

Output Function (optional)

⚙️ Optional function to apply to the result before returning it (e.g., str => str + ' €', val => val !== undefined for boolean conversion). Useful for chaining transformations on the output.

Date String string

💡 Type any text to see the result update live

🎯 Using special value — click input to type instead

Test with:

Falsy undefined null "" empty 0 false NaN

Truthy true 42 "hello" {} empty [] empty {a:1} [1,2,3]

🔗 Result Handling — Chain Variables

Chain apply-mode variables to the output. Each variable receives the result of the previous one.

getDayIndexFromDateString()

+ Chain another variable

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Related Variables

Same category: Date

→ addDays → basicDateToExtended → calculateDaysDifference → calculateLeadTime → currentTimestamp → dateToUnixTimestamp → daysToMilliseconds → extendedDateToBasic → getDayName → getDayNameFromDateString → getDayNameShort → getHourOfDay → getMonthName → getMonthNameShort → hoursToMilliseconds → isBusinessHours → isWeekend → millisecondsToDays → millisecondsToHours → millisecondsToMinutes → millisecondsToSeconds → minutesToMilliseconds → reverseDateCompactDay1st → reverseDateCompactYear1st → reverseDateSlash → secondsToMilliseconds → subtractDays → timestampToDayIndex → unixTimestampToDate

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Under the Hood

📜 View Implementation Code

/**
* Returns the day index corresponding to an ISO 8601 date string using Zeller's Congruence algorithm.
*
* @param {string} data.src - A date string in YYYY-MM-DD format.
* @param {Function|string} [data.out] - Optional output handler: function to transform result or string with format.
*
* Direct-mode specific parameters:
* @param {Function} [data.pre] - Optional pre-processor function to transform src before calculation.
*
* @returns {number|undefined} The index of the day, or undefined if input is invalid.
*
* @framework ggLowCodeGTMKit
*/
const Math = require('Math');
const makeNumber = require('makeNumber');

const getDayIndexFromDateString = function(dateStr) {
   const reDateFormatISO8601 = "(\\d\\d\\d\\d)-(\\d\\d)-(\\d\\d)";
   const dateMatchParts = dateStr.match(reDateFormatISO8601);
   if (dateMatchParts === null) { return undefined; }

   let year = makeNumber(dateMatchParts[1]);
   let month = makeNumber(dateMatchParts[2]);
   let day = makeNumber(dateMatchParts[3]);

   // Zeller's Congruence algorithm
   if (month < 3) {
       month = month + 12;
       year = year - 1;
   }
   const q = day;
   const m = month;
   const k = year % 100;
   const j = Math.floor(year / 100);
   const h = (q + Math.floor((13 * (m + 1)) / 5) + k + Math.floor(k / 4) + Math.floor(j / 4) + 5 * j) % 7;
   // Convert Zeller (0=Sat) to ISO 8601 (1=Mon, 7=Sun)
   return ((h + 5) % 7) + 1;
};

const safeFunction = fn => typeof fn === 'function' ? fn : x => x;
const out = safeFunction(data.out);

// ===============================================================================
// getDayIndexFromDateString - Direct mode
// ===============================================================================
const applyCast = (castFn, value) => safeFunction(castFn)(value);
const value = applyCast(data.pre, data.src);
return out(getDayIndexFromDateString(value));
// ===============================================================================
// getDayIndexFromDateString() – Apply Mode
// ===============================================================================
/*
return function(value) {
  return out(getDayIndexFromDateString(value));
};
*/

🧪 View Test Scenarios (5 tests)

✅ '[example] Saturday returns 6'
✅ '[example] Monday returns 1'
✅ Valid ISO date with January - handles month adjustment correctly
✅ '[example] Invalid format returns undefined'
✅ Invalid string - returns undefined