Adding values in ASP.NET controller ​from the database SQL server column for a specific user

Id | Login | Year | Month | Day | Overtime
---+-------+------+-------+-----+----------
 1 | MIPA  | 2020 |   1   |  1  |  08:00
 2 | MIPA  | 2020 |   1   |  2  |  08:00
 3 | MIPA  | 2020 |   1   |  3  |  07:30
 4 | MIPA  | 2020 |   1   |  4  |  12:00
...
30 | MIPA  | 2020 |   1   |  30 |  04:00
...
41 | MIPA  | 2020 |   2   |  1  |  08:00
42 | MIPA  | 2020 |   2   |  2  |  08:00
43 | MIPA  | 2020 |   2   |  3  |  07:30
44 | MIPA  | 2020 |   2   |  4  |  12:00
...
52 | MIPA  | 2020 |   2   |  25 |  04:00

[HttpPost]
public async Task<ActionResult> PartialTableEcp()  //change nothing
{
// Data is already extracted from other operations
// So e.g. user enters chooses year: 2020 month: March (3) 
int numberMonth= 3;
int numberYear= 2020;
int numberOfDays= 31;

// person identification by login
var userName = _httpContextAccessor.HttpContext.User.Identity.Name;

// retrieve all month records from the table, (i dont know it works)
var existingRecords = _tableEcpContext.Karta
                                      .Where(x => x.Login == userName && 
                                                  x.Year == numberYear && 
                                                  x.Month == numberMonth-1) ////will mathematical calculation work here?
                                      .Select(i => new{
                                                       i.Overtime
                                                       }).ToList(); 


and now there is a problem how to sum all these values ​​.. preferably in HHH: mm format


var thisValue = that this value would be written to the variable var;

var viewModel = new ParentView { Model1 = karta };
return PartialView("_TableViewEcp", viewModel); //change nothing

}

public partial class Karta_Model
    {
        [Key]
        public int Id { get; set; }
        public string? Login { get; set; }
        public int Year { get; set; }
        public int Month{ get; set; }
        public int? DayOfMonth { get; set; }
        public string? Overtime { get; set; }
     }

public partial class ParentView
    {
        public List<Karta_Model> Model1 { get; set; }
    }