Express Generator calculates the backtest on all of the provided data by default.
We have several means to control the data set.
1) Limit the count of the data bars:
We can do it when fetching the data or when running the Generator.
> node .\bin\fetch.js --server Eightcap --symbol EURUSD --period M30 --max-data-bars 100000
..:: Express Generator Fetch v2.65 ::..
Fetched: Eightcap EURUSD M30, From: 2017-03-08 00:00, To: 2025-03-21 22:30, Bars: 100000
> node .\bin\gen.js --server Eightcap --symbol EURUSD --period M30
We see that the Generator uses the complete data set.

We obtained the same result by downloading the complete data set and limiting the data bars when running the generator.
> node .\bin\fetch.js --server Eightcap --symbol EURUSD --period M30
..:: Express Generator Fetch v2.65 ::..
Fetched: Eightcap EURUSD M30, From: 2009-02-04 17:00, To: 2025-03-21 22:30, Bars: 200000
Data fetch ready!
> node .\bin\gen.js --server Eightcap --symbol EURUSD --period M30 --max-data-bars 100000
2) Limit the data's start and end dates.
We have four parameters to control the start and end dates of the data set. The
use_data_start = false
data_start = "01 Jan 2020 00:00 UTC"
use_data_end = false
data_end = "30 Jun 2023 00:00 UTC"
We enable the corresponding limitation and set the desired date and time.
Usually, we limit the "data end" and use the count of bars limitation.

( I'm using "backtick `" and shift+enter to make a new line)
We see that the Generator loads 100,000 bars ending at 2024-12-31 23:00.
We can also make it by setting the count of days for the data end. This is useful because we can have it in a settings file, which will work with the time.
PS C:\express-generator> node .\bin\gen.js --server Eightcap --symbol EURUSD --period M30 `
>> --use-data-end true --data-end -90 --max-data-bars 100000
--use-data-end true --data-end -90 - it sets the Generator to load data up to 90 days back from the current time.
3) Out of sample
We have a convenient way to cut part of the data.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Out of sample ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Use these parameters to set the data range.
; The program cuts data after applying Data Horizon.
data_start_percent = 0
data_end_percent = 100
The out-of-sample parameters are applied after the data-sart, data-end, and max-bars limitations.
Here is how to have the oldest 80% of the data
PS C:\express-generator> node .\bin\gen.js --server Eightcap --symbol EURUSD --period M30 `
>> --data-start-percent 0 --data-end-percent 80
..:: Express Generator v2.65 ::..
Market : Eightcap EURUSD M30
From : 2017-03-08 00:00, To: 2023-08-11 06:30, Bars: 80000
Spread : 20, Swap long: -7.44, Swap short: 2.9, Commission: 6 USD
Account: 10000 USD, Leverage: 100, Entry: 0.01 lots