var myDate=new DateTime(2018, 1, 1)

...
where o.PostingDate <= myDate

SELECT [o].[EntryNo], [o].[AccountNo], [o].[Amount], [o].[BalAccountNo], [o].[BalAccountType], [o].[Description], [o].[DocumentDate], [o].[DocumentNo], [o].[PostingDate], [o].[SourceNo], [o].[SourceType]
FROM [PR_GLEntries] AS [o]
WHERE [o].[PostingDate] <= '2018-01-01T00:00:00.0000000'
ORDER BY [o].[AccountNo]

            var gr = (from o in db.PR_GLEntries
                      group o by o.AccountNo into os
                      select new { AccNo = os.Key, Sum = os.Sum(o => o.Amount) }).ToList();

SELECT [o].[AccountNo] AS [AccNo], SUM([o].[Amount]) AS [Sum]
FROM [PR_GLEntries] AS [o]
GROUP BY [o].[AccountNo]

OverflowException: Conversion overflows.

    System.Data.SqlClient.SqlBuffer.get_Decimal()
    System.Data.SqlClient.SqlBuffer.get_Value()
    System.Data.SqlClient.SqlDataReader.GetFieldValueFromSqlBufferInternal<T>(SqlBuffer data, _SqlMetaData metaData)
    System.Data.SqlClient.SqlDataReader.GetFieldValue<T>(int i)
    lambda_method(Closure , DbDataReader )
    Microsoft.EntityFrameworkCore.Storage.Internal.TypedRelationalValueBufferFactory.Create(DbDataReader dataReader)
    Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable+Enumerator.BufferlessMoveNext(DbContext _, bool buffer)
    Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.Execute<TState, TResult>(TState state, Func<DbContext, TState, TResult> operation, Func<DbContext, TState, ExecutionResult<TResult>> verifySucceeded)
    Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable+Enumerator.MoveNext()
    Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider+ExceptionInterceptor+EnumeratorExceptionInterceptor.MoveNext()
    System.Collections.Generic.List.AddEnumerable(IEnumerable<T> enumerable)
    System.Linq.Enumerable.ToList<TSource>(IEnumerable<TSource> source)
    AccountPlane.Controllers.HomeController.Index(Nullable<DateTime> ReportDate) in HomeController.cs

                var gr = (from o in db.PR_GLEntries

            var gr = (from o in db.PR_GLEntries
                      group o by o.AccountNo into os
                      select new { AccNo = os.Key, Sum = os.Sum(o => (double)o.Amount) }).ToList();