How to filter out the records in datatable for particular column having dates older than last 90 days?

we keep in mind:

Assign Activity:
dtFiltered =

(From d in YourDataTableVar.asEnumerable
Let dp = DateTime.ParseExact(row("Last Logon").ToString(), "M/d/yyyy", System.Gobalization.CultureInfo.InvariantCulture)
Where dp.Date < DateTime.Now.AddDays(-90).Date
Select r =d).CopyToDataTable

As mentioned above within the FirstAid we check the DataTable and align the DateTime parsing accordingly

Handling empty results:

Feel free to adapt the LINQ e.g. by handling empty row values / non parseable values etc