GhazanfarApril 16, 20262 min read

Your Looker Studio dashboard shows a 45% year-over-year increase in revenue — but when your finance team checks the same date range in GA4, they see only 12% growth. This discrepancy usually comes from how Looker Studio handles date range comparisons: partial weeks, different day counts, timezone mismatches, and comparison period logic errors. This guide explains why these mismatches happen and how to build accurate date comparisons.

Common Causes of Wrong Comparison Numbers

  • Day Count Mismatch: Comparing Feb 1-28 this year vs Feb 1-28 last year, but last year was a leap year with 29 days.
  • Partial Period: Comparing “this month” (March 1-16) to “last month” (February 1-28) — 16 days vs 28 days.
  • Timezone Difference: GA4 property in Eastern Time but Looker Studio data source in UTC creates different time boundaries.
  • Week Definition: GA4 starts weeks on Sunday; your finance system starts on Monday.

Setting Up Correct Date Controls

  • Always use explicit date range controls, not “this month” or “last month” presets
  • For YoY: set primary range to Jan 1–Dec 31, comparison to Jan 1–Dec 31 previous year
  • For MoM: compare only completed months, not partial current month
  • Add a text note showing exact dates being compared
// Looker Studio calculated field for proper YoY same-period comparison
// Create field "Is Current Year": YEAR(Date) = YEAR(TODAY())
// Create field "Day of Year": DAYOFYEAR(Date)

// Filter current year: Is Current Year = True AND Day of Year <= DAYOFYEAR(TODAY())
// Filter prior year: YEAR(Date) = YEAR(TODAY())-1 AND Day of Year <= DAYOFYEAR(TODAY())
// This ensures same number of days in each year comparison

Accurate YoY with Blended Data

Looker Studio Date Range Comparisons: Why YoY and MoM Numbers Are Wrong and How to Fix
  • Data source 1: GA4, date range "Current Year to Date"
  • Data source 2: Same GA4, date range "Prior Year to Date"
  • Add calculated field to Source 2: "Adjusted Date" = DATE_ADD(Date, INTERVAL 1 YEAR)
  • Blend on Adjusted Date = Date — this joins Jan 5, 2024 (adjusted) with Jan 5, 2025

Timezone Alignment

Go to your GA4 data source in Looker Studio and check the timezone setting — it should match your GA4 property timezone. If GA4 property is Eastern Time, set Looker Studio data source to Eastern Time. Never mix UTC data with local-time data in the same comparison.

Validation Checklist

  • ☐ Check day count: comparison periods have the same number of days
  • ☐ Check timezone: both data sources use the same timezone
  • ☐ Verify with raw GA4: spot-check 3 specific dates in Looker Studio vs GA4 directly
  • ☐ For YoY: ensure you're not including partial current period vs complete prior period
  • ☐ Document comparison methodology in dashboard description

Accurate date comparisons require explicit, defensible date range definitions. When someone asks "why are these numbers different from last year?", you should be able to show exactly which days are being compared.

Related guides: Looker Studio GA4 Data Freshness, Looker Studio Blended Data Sources, Looker Studio Audience Filters.

Guide: Looker Studio Date Range Comparisons: Why YoY and

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