How time works in a retention analysis

In a Retention Analysis chart, there are two ways to think about what a “day” is: as a series of rolling 24-hour windows or as strict calendar dates. The method you choose can affect your results.

Amplitude treats a day as a rolling 24-hour window by default, which is different for each user. The beginning of the 24-hour window starts when a user triggers the starting event (Day Zero). Day 1 runs from hour 24 to hour 48, Day 2 from hour 48 to hour 72, and so on. Each day will be exactly the same length, no matter when the user triggered the starting event. 

When using strict calendar dates, daily calendar dates start when the calendar day starts, and end when the calendar day ends. The calendar view is determined by the timezone specified in your project settings. Under the strict calendar view, daily retention is based on the calendar day instead of on an hourly basis. 

Daily retention using 24-hour windows

Daily retention is computed on an hourly basis. 

When Amplitude calculates daily retention, it rounds event timestamps down to the most recent hour. This means an event triggered at 5:59 pm will have a timestamp of 5:00 pm.

A user is counted as next-day retained if they trigger any event during or after the 24th-incremented hour, but before the 48th-incremented hour. For instance, if a user triggers their first event on December 1st at 5:59 PM, and then a second event on December 2nd at 5:00 PM, Amplitude counts that user as Day 1 retained (instead of Day Zero), because that is the 24th-incremented hour after the timestamp on the original event.

When a user triggers the initial event multiple times, Amplitude will start multiple 24-hour buckets for them. This means it’s possible for one return event to define a user as both Day 1 and Day 2 retained.

Weekly and monthly retention using 24-hour windows 

Weekly and monthly retention is computed on a daily basis.

A week is defined as seven days. If a user triggers their first event on December 1st, Amplitude considers them as Week Zero retained if they trigger an event on any day between December 1st and December 7th. The user will be considered Week 1 retained if they trigger an event on any day between December 8th and December 14th.

A month is defined as 30 days. If a user triggers their first event on December 3rd, Amplitude considers them as Month Zero retained if they trigger the return event on any day between December 3rd and January 2nd. The user will be considered Month 1 retained if they trigger the return event on any day between January 3rd and February 2nd.

NOTE: Any retention computations that include dates of August 17, 2015 or earlier will be computed by calendar time instead.

Amplitude can also measure retention by strict calendar dates, where day X is measured from the calendar date the event was triggered.

Daily retention using strict calendar dates

Daily calendar dates start when the calendar day starts, and end when the calendar day ends.

The calendar view is determined by the timezone specified in your project settings. Under the strict calendar view, daily retention is based on the calendar day instead of on an hourly basis. 

A user is counted as next-day retained if they fire any event during the next calendar day. For instance, if a user triggers their first event on December 1st at 5:59 PM, Amplitude considers them as Day 1 retained if they trigger an event any time before December 2nd at 11:59 PM. If a user triggered their first event on December 1st at 11:50 PM, they have until December 1st at 11:59 PM to trigger the return event and still be counted as Day Zero retained. 

Users who trigger the starting event multiple times are still restricted to the calendar day they first triggered the starting event. The exception to this is when a user triggers the starting events on multiple calendar days; in this case, that user will be included in multiple interval cohorts.

Weekly and monthly retention using strict calendar dates 

Weekly and monthly calendar dates determine the beginning and end of each week and month.

A week is defined based on the timezone specified in your project settings. There you’ll also find the option to specify the first day of your week. 

If your weeks start on Mondays and a user triggers their first event on Monday, December 1st, Amplitude considers the user as Week Zero retained if they trigger an event on any day between December 1st and December 7th. Amplitude will consider that user as Week 1 retained if they trigger an event on any day between December 8th and December 14th. Another user who triggers their first event on Tuesday, December 2nd has until December 7th to trigger the return event in order to be Week Zero retained. 

The definition of a month is based on the timezone specified in your project settings. If a user triggers their first event on December 3rd, Amplitude considers them as Month Zero retained if they trigger the return event on any day between December 3rd and December 31st, and Month 1 retained if the user triggers the return event on any day between January 1st and January 31st.

NOTE: For new user retention, filter conditions applied in the Segmentation module are only satisfied if they are true during the same time frame the new user event was triggered. For charts using strict calendar dates, this is the same as the chart interval. For charts using unaligned ranges, the time frame is more granular: e.g., the first day for seven-day windows and the first hour for 24-hour windows.

This table further delineates the differences between 24-hour windows and strict calendar dates.