scope calling user-defined C# methods in #DECLARE statement

Question

Is it possible to use a user-defined C# method in a #DECLARE statement? I have some code like this, but it doesn't seem to work.

#DECLARE timespan int = 120;
#DECLARE endDate string = @"2022-03-09";
#DECLARE startDate string = GetStartDate(@endDate, @timespan);

...

#CS
public static string GetStartDate(string endDateString, int daysBefore)
    {
        TimeSpan timespan = new TimeSpan(daysBefore, 0, 0, 0);
        DateTime endDate = DateTime.Parse(endDateString);
        DateTime startDate = endDate - timespan;

        var startDateString = startDate.ToString("u", CultureInfo.GetCultureInfo("en-US"));
        startDateString = startDateString.Split(' ')[0];
        return startDateString;
    }
#ENDCS