How to get Sql datediff() function in linq

Please, read my comment to the question. We can't "convert" your sql query to Linq query without seeing the data.

Have a look here:
SqlMethods.DateDiffDay Method[^]
LINQ to SQL Tips and Tricks[^]

[EDIT 2]

var qry = from a in md
        group a by new{Year = a.RecDate.Year, Month = a.RecDate.ToString("MMM"), Dept = a.RelDept} into grp
        select new
        {
            Year = grp.Key.Year,
            Month = grp.Key.Month,
            Dept = grp.Key.Dept,
            AvgIA = grp.Average(g=>(g.RecDate - (g.Intl_Ack !=null ? g.Intl_Ack : DateTime.Today)).Days),
            AvgRD = grp.Average(g=>(g.RecDate - (g.Resol_Date !=null ? g.Resol_Date : DateTime.Today)).Days),
            SumOp = grp.Sum(g=>g.Status=="Open" ? 1 : 0)
        };

Solution #2 Not entirely LinqToSql
It only works because of ToList(). ToList() like AsEnumerable() causes the rest of the expression to be evaluated client-side; This is Linq, but not LinqToSql and may be unacceptable for large result sets.

ExecuteQuery
One trick if already have the SQL, is to use something like:
dataContext.ExecuteQuery<tresult>(query, optionalParameterValues);

You will have to work out what to use for TResult.

LinqToSql Executes SQL on the Server
LinqToSql takes some C# expressions & functions and maps those to the SQL so that the expression is evaluated on the server. Not all expressions can be translated, resulting in run-time errors. The solution is to find what C# can be translated into SQL code and the best place server/client to execute it.

In the OP’s case since a "group by" is done, it may actually be more efficient to force the query client-side. LinqToSql sometimes processes "group by" as n queries where n is the number of groups.

Some methods cannot be mapped to SQL. Other expressions may appear to be mapped to SQL and run on the host, but, they aren’t; for example, values that can be determined before the query on the client. So you may be able to use DateTime.Add, when the operands are available before the query, but not on a column value.

Often a little experimentation can find the C# that will work. In this case, it turns out to be easier than expected, because while Date.Add cannot be mapped, other expressions with dates can. Here some examples:

DateTime SqlDateMin = new DateTime(1999, 1, 1);
DateTime SqlDateMax = new DateTime(9999, 12, 31);

var query3 =
from ue in Company_Diff
let startDate = ue.SysRowStartDate == null ? SqlDateMin : ue.SysRowStartDate
let endDate = ue.SysRowEndDate == null ? SqlDateMax : ue.SysRowEndDate
let next = startDate + TimeSpan.FromDays(1)
let year = endDate.Value.Year
let diff = ue.SysRowEndDate - startDate

select new { ue.SysRowEndDate, ue.SysRowStartDate, diff, next, year };


You need to handle null dates, or you will get exceptions when you query data with nulls and attempt to add.
Again, to see the resulting SQL use LinqPad and view it on the SQL table.