How to get all model validation errors in ASP.NET api

My model looks like this:

public class Meeting : IValidatableObject
{
    public string Id { get; set; }
    [Required]
    public string Start_time { get; set; }
    [Required]
    public string End_time { get; set; }
    [Required]
    public DateTime Date { get; set; }
    [Required]
    [Url]
    public string Url { get; set; }
    [Required]
    [RegularExpression(@"^[(a-zA-Z)' '(a-zA-Z)]*$",
        ErrorMessage = "Characters are not allowed.")]
    public string Owner { get; set; }
    [Required]
    [RegularExpression(@"^[(a-zA-Z)' '(a-zA-Z)]*$",
        ErrorMessage = "Characters are not allowed.")]
    public string Participant { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        List<ValidationResult> results = new List<ValidationResult>();
        DateTime dt, dt1;
        if(DateTime.TryParse(Start_time, out dt) == false)
        {
            results.Add(new ValidationResult("Start time wrong", new[] { "Start time" }));
        }
        if (DateTime.TryParse(End_time, out dt1) == false)
        {
            results.Add(new ValidationResult("End time wrong", new[] { "End time" }));
        }
        if (string.Compare(Start_time, End_time) == 1)
        {
            results.Add(new ValidationResult("Time Conflict", new[] { "Time" }));
        }
        return results;
    }
}

And I don't know how to get all errors in JSON both from model itself and IEnumerable<ValidationResult> Validate method

My POST method has this realization:

    [HttpPost]
    public IActionResult Create([FromBody] Meeting meet)
    {
        if (ModelState.IsValid)
        {
            Guid obj = Guid.NewGuid();
            meet.Id = obj.ToString();
            _dataAccessProvider.AddMeetingRecord(meet);
            return Ok("Meeting was successfully added");
        }
        return BadRequest(ModelState);
    }

I tried to use IEnumerable allErrors = ModelState.Values.SelectMany(v => v.Errors); and return allErrors in BadRequest but it didn't work