I'm afraid, Specflow provides this feature. I'd a similar requirement in past and we have used the below code (taken from here) which converts string to proper datetime.
- next month, next year,
- next 4 months, next 3 days
- 3 days ago, 5 hours ago
- tomorrow, yesterday
- last year, last month,
- last tue, next fri
- last june, next may, 
- jan 2008, 01 january 2009,
- june 2019, 2009/01/01
The helper class:
class FuzzyDateTime
{
    static List<string> dayList = new List<string>() { "sun", "mon", "tue", "wed", "thu", "fri", "sat" };
    static List<IDateTimePattern> parsers = new List<IDateTimePattern>()
    {
       new RegexDateTimePattern (
            @"next +([2-9]\d*) +months",
            delegate (Match m) {
                var val = int.Parse(m.Groups[1].Value); 
                return DateTime.Now.AddMonths(val);
            }
       ),
       new RegexDateTimePattern (
            @"next +month",
            delegate (Match m) { 
                return DateTime.Now.AddMonths(1);
            }
       ),           
       new RegexDateTimePattern (
            @"next +([2-9]\d*) +days",
            delegate (Match m) {
                var val = int.Parse(m.Groups[1].Value); 
                return DateTime.Now.AddDays(val);
            }
       ),
       new RegexDateTimePattern (
            @"([2-9]\d*) +months +ago",
            delegate (Match m) {
                var val = int.Parse(m.Groups[1].Value); 
                return DateTime.Now.AddMonths(-val);
            }
       ),
       new RegexDateTimePattern (
            @"([2-9]\d*) days +ago",
            delegate (Match m) {
                var val = int.Parse(m.Groups[1].Value); 
                return DateTime.Now.AddDays(-val);
            }
       ),
       new RegexDateTimePattern (
            @"([2-9]\d*) *h(ours)? +ago",
            delegate (Match m) {
                var val = int.Parse(m.Groups[1].Value); 
                return DateTime.Now.AddMonths(-val);
            }
       ),
       new RegexDateTimePattern (
            @"tomorrow",
            delegate (Match m) {
                return DateTime.Now.AddDays(1);
            }
       ),
       new RegexDateTimePattern (
            @"today",
            delegate (Match m) {
                return DateTime.Now;
            }
       ),
       new RegexDateTimePattern (
            @"yesterday",
            delegate (Match m) {
                return DateTime.Now.AddDays(-1);
            }
       ),
       new RegexDateTimePattern (
            @"(last|next) *(year|month)",
            delegate (Match m) {
                int direction = (m.Groups[1].Value == "last")? -1 :1;
                switch(m.Groups[2].Value) 
                {
                    case "year":
                        return new DateTime(DateTime.Now.Year+direction, 1,1);
                    case "month":
                        return new DateTime(DateTime.Now.Year, DateTime.Now.Month+direction, 1);
                }
                return DateTime.MinValue;
            }
       ),
       new RegexDateTimePattern (
            String.Format(@"(last|next) *({0}).*", String.Join("|", dayList.ToArray())), //handle weekdays
            delegate (Match m) {
                var val = m.Groups[2].Value;
                var direction = (m.Groups[1].Value == "last")? -1 :1;
                var dayOfWeek = dayList.IndexOf(val.Substring(0,3));
                if (dayOfWeek >= 0) {
                    var diff = direction*(dayOfWeek - (int)DateTime.Today.DayOfWeek);
                    if (diff <= 0 ) { 
                        diff = 7 + diff;
                    }
                    return DateTime.Today.AddDays(direction * diff);
                }
                return DateTime.MinValue;
            }
       ),
       new RegexDateTimePattern (
            @"(last|next) *(.+)", // to parse months using DateTime.TryParse
            delegate (Match m) {
                DateTime dt;
                int direction = (m.Groups[1].Value == "last")? -1 :1;
                var s = String.Format("{0} {1}",m.Groups[2].Value, DateTime.Now.Year + direction);
                if (DateTime.TryParse(s, out dt)) {
                    return dt;
                } else {
                    return DateTime.MinValue;
                }
            }
       ),
       new RegexDateTimePattern (
            @".*", //as final resort parse using DateTime.TryParse
            delegate (Match m) {
                DateTime dt;
                var s = m.Groups[0].Value;
                if (DateTime.TryParse(s, out dt)) {
                    return dt;
                } else {
                    return DateTime.MinValue;
                }
            }
       ),
    };
    public static DateTime Parse(string text)
    {
        text = text.Trim().ToLower();
        var dt = DateTime.Now;
        foreach (var parser in parsers)
        {
            dt = parser.Parse(text);
            if (dt != DateTime.MinValue)
                break;
        }
        return dt;
    }
}
interface IDateTimePattern
{
    DateTime Parse(string text);
}
class RegexDateTimePattern : IDateTimePattern
{
    public delegate DateTime Interpreter(Match m);
    protected Regex regEx;
    protected Interpreter inter;
    public RegexDateTimePattern(string re, Interpreter inter)
    {
        this.regEx = new Regex(re);
        this.inter = inter;
    }
    public DateTime Parse(string text)
    {
        var m = regEx.Match(text);
        if (m.Success)
        {
            return inter(m);
        }
        return DateTime.MinValue;
    }
}
Usage example:
var val = FuzzyDateTime.Parse(dateString);
And the following user data
|User_ID | Start_Date | End_Date|
|ABCX123 |tomorrow  |next month|