Как определить первый и последний день текущего месяца?

declare @startdate date, @enddate date;
 
select
 @startdate = dateadd(day, -day(getdate()) + 1, cast(getdate() as date)),
 @enddate = dateadd(day, -1, dateadd(month, 1, @startdate));
 
-- Для 2012+
--select
-- @enddate = eomonth(@startdate);
 
select @startdate, @enddate, eomonth(@startdate);

declare
@startdate date, 
@enddate date
 
SET @startdate = dateadd(month,datediff(month,0,GetDate()),0);
SET @enddate =  dateadd(day, -1, dateadd(month,1, @startdate));

declare
@startdate date, 
@enddate date

declare
@startdate datetime, 
@enddate datetime

SELECT t1._Fld7641 AS [Дата/Время], t1._Fld7643 AS [Ср.время], t2._Description AS [Описание], t1._Fld7647 AS [ФИО]
FROM dbo._InfoRg7639 AS t1
JOIN dbo._Reference5145 AS t2 ON t1._Fld7640RRef = t2._IDRRef
WHERE t1._Fld7641>=DATEADD(MONTH,  DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0)
  AND t1._Fld7641< DATEADD(MONTH,1+DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0);

SELECT t1._Fld7641 AS [Дата/Время], t1._Fld7643 AS [Ср.время], t2._Description AS [Описание], t1._Fld7647 AS [ФИО]
FROM dbo._InfoRg7639 AS t1
JOIN dbo._Reference5145 AS t2 ON t1._Fld7640RRef = t2._IDRRef

SELECT t1._Fld7641 AS [Дата/Время], t1._Fld7643 AS [Ср.время], t2._Description AS [Описание], t1._Fld7647 AS [ФИО]
FROM dbo._InfoRg7639 AS t1
JOIN dbo._Reference5145 AS t2 ON t1._Fld7640RRef = t2._IDRRef
WHERE t1._Fld7641>=CAST(DATEADD(MONTH,  DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0) AS DATETIME2)
  AND t1._Fld7641< CAST(DATEADD(MONTH,1+DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0) AS DATETIME2);

SELECT t1._Fld7641 AS [Дата/Время], t1._Fld7643 AS [Ср.время], t1._Fld7647 AS [ФИО]
FROM dbo._InfoRg7639 AS t1
WHERE t1._Fld7641>=CAST(DATEADD(MONTH,  DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0) AS DATETIME2)
  AND t1._Fld7641< CAST(DATEADD(MONTH,1+DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0) AS DATETIME2);

set statistics xml on;
 
Ваш проблемный запрос
 
set statistics xml off;

set statistics xml on;
 
SELECT t1._Fld7641 AS [Дата/Время], t1._Fld7643 AS [Ср.время], t2._Description AS [Описание], t1._Fld7647 AS [ФИО]
FROM dbo._InfoRg7639 AS t1
JOIN dbo._Reference5145 AS t2 ON t1._Fld7640RRef = t2._IDRRef
WHERE t1._Fld7641>=CAST(DATEADD(MONTH,  DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0) AS DATETIME2)
  AND t1._Fld7641< CAST(DATEADD(MONTH,1+DATEDIFF(MONTH,0,CURRENT_TIMESTAMP),0) AS DATETIME2);
 
set statistics xml off;