Subtract one month from Datetime.Today

Patrick got it. To build on his answer and improve error handling (if there's a possibility that qs could be an invalid date string), you might do something like:

DateTime qsValue;

dateTimePicker1.MaxDate = DateTime.Today.AddMonths(-1);

dateTimePicker1.Value = (DateTime.TryParse(qs, out qsValue))
    ? qsValue
    : dateTimePicker1.MaxDate;

Just substract a month by 'adding` -1:

var lastmonth = DateTime.Today.AddMonths(-1);

See the MSDN documentation on DateTime.AddMonths.