Nth day dimension
Nth day is the dimension that numbers each day by its offset from the start of the selected date range — day 0, day 1, and so on — rather than by calendar date. This relative indexing is what lets you overlay two periods of equal length so their day 1s align, the standard technique for period-over-period and launch-cohort comparison regardless of the actual calendar dates involved.
What this means
Nth day replaces the absolute calendar date with a relative index counted from the first day of the chosen range. Two periods of the same length both run day 0 to day N, so plotting them together aligns equivalent positions regardless of their calendar dates.
This is the dimension behind 'this period vs last period' overlays.
Range-relative meaning
Because the index is defined by the report's date range, the same calendar day maps to different nth-day values under different ranges, and the numbering resets when you change the range. It carries no fixed meaning outside the report it was computed in.
Use it strictly for relative alignment, and use the date dimension when you need a fixed calendar anchor.
- Offset in days from the range start (0,1,2…)
- Aligns equal-length periods for overlay
- Numbering depends on the selected range
How it appears in analytics and logs
An nth day value is the offset from the range start, not a calendar date. Its numbering shifts when you change the date range, so it is meaningful only relative to that range.
Diagnostic use case
Use nth day to align two equal-length periods so their day-one values overlay, enabling clean period-over-period or post-launch comparisons.
What WebmasterID can help detect
WebmasterID can express timing as days-since-baseline first-party, supporting aligned period comparisons without third-party tracking.
Common mistakes
- Treating nth day as a fixed calendar date.
- Comparing nth-day series from unequal-length ranges.
- Forgetting the index resets when the range changes.
Privacy and accuracy notes
Nth day records a relative day index, not identity. WebmasterID can index days relative to a baseline first-party without profiling visitors.
Related pages
- Nth week dimension
Nth week is the dimension that numbers weeks by their offset from the start of the selected range — week 0, week 1, and so on. It is the weekly analog of nth day, built for multi-week retention curves and cohort decay where you want week-1 retention to align across cohorts regardless of when each began. Its numbering is relative to the report's range and resets when the range changes.
- Date dimension
Date is the dimension that records the calendar day of an event in YYYYMMDD form, computed in the GA4 property's reporting time zone. It is the default x-axis of most trend reports. Its day boundary is the property's midnight, not UTC or the visitor's clock, so the same raw event can be assigned to a different date than another tool would choose.
- Cohort dimension
The cohort dimension groups users by a shared starting point — typically their acquisition date — so you can follow each group's behaviour across subsequent days, weeks, or months. GA4 builds cohorts in the Cohort exploration from a first-touch criterion and a return criterion. It is the backbone of retention analysis, but small cohorts and identity loss can make later-period values unstable, so trends matter more than single cells.
- Web analytics
Aligned period-over-period comparison first-party.
Sources and verification notes
- Google Analytics Help — [GA4] Dimensions, metrics, and other termsDefines nth-day as an offset from the range start.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.