You can tell the server to only show data from some date on or for the last X days. So if you want to have only last 90 days of data, you need to modify ManicTimeServerSettings.json file, so that it includes:
{
....
"activityReport": {
"maxAge": 90
}
....
}
You can also set it to keep the data from some date on, so for example if you want to have the data from 1/1/2021, you would modify ManicTimeServerSettings.json like this:
{
....
"activityReport": {
"minDate": "2021-01-01"
}
....
}
You could use both settings:
{
....
"activityReport": {
"minDate": "2021-01-01",
"maxAge": 90
}
....
}
In this case the most restrictive would win.