Sample code of using a Regular Expression to turn a string representation of an excel-like date formula into an actual date value.

labs_ParseDateFormula.cs

public string EvaluateDateCalculation(string dateValue)
{
    string dateFormula = @"(?:([0-9]+)[\s]*([+-])[\s]*)?(TODAY\(\))(?:[\s]*([+-])[\s]*([0-9]+))?";
    System.Text.RegularExpressions.Regex dateRegex = new System.Text.RegularExpressions.Regex(dateFormula);

    System.Text.RegularExpressions.Match dateMatch = dateRegex.Match(dateValue);
    if (dateMatch.Success)
    {
        System.Text.RegularExpressions.GroupCollection calcGroups = dateMatch.Groups;

        // Find the base date (Right now, only TODAY() is supported, but something like DATE(1, 1, 2021) might be supported later)
        var baseDateGroup = calcGroups[3];
        DateTime dateToReturn = new DateTime();
        if (baseDateGroup.Value == "TODAY()")
        {
            dateToReturn = DateTime.Now;
        }

        dateToReturn = HandleDateChange(dateToReturn, calcGroups[1], calcGroups[2]);
        dateToReturn = HandleDateChange(dateToReturn, calcGroups[5], calcGroups[4]);

        // In order to have this actually be useful as a query, we need to set up a condition to get all dates that lie between
        // Midnight and 11:59 PM (or 23:59) of the selected date
        string midnightDateFormat = "yyyy-MM-ddT00:00:00";
        string lastMinuteFormat = "yyyy-MM-ddT23:59:59";
        return $"{dateToReturn.ToString(midnightDateFormat)} and {dateToReturn.ToString(lastMinuteFormat)}";
    }

    // If our regular expression did not match anything, then that means 
    // this is just a normal date that we can pass along without modification
    return dateValue;
}

public DateTime HandleDateChange(DateTime date, System.Text.RegularExpressions.Group numDaysGroup, System.Text.RegularExpressions.Group modifierGroup)
{
    if (numDaysGroup.Success && modifierGroup.Success)
    {
        int numDays = Int32.Parse(numDaysGroup.Value);
        string modifier = modifierGroup.Value;

        return date.AddDays((modifier == "+") ? numDays : (numDays * -1));
    }

    // Just return the base date if the groups were not found
    return date;
}