SELECT * FROM  OPENQUERY([MYSQLCONN],
        'SELECT e_id, TDate, Entry, `Exit` FROM timemgt.daymaster
        WHERE TDate >= ''''' + @frmDate + ''''' ')

SELECT * FROM  OPENQUERY([MYSQLCONN],
    'SELECT e_id, TDate, TIME_FORMAT(Entry, ''%T'') AS ''Entry'',
    TIME_FORMAT(`Exit`, ''%T'') AS ''Exit'' FROM timemgt.daymaster
    WHERE TDate >= ''2017-01-01'' AND TDate <= ''2017-01-01''')

DECLARE @frmDate VARCHAR(10)
DECLARE @toDate VARCHAR(10)
SET @frmDate = '2017-01-01'
SET @toDate = '2017-01-01'

SELECT * FROM  OPENQUERY([MYSQLCONN],
    'SELECT e_id, TDate, TIME_FORMAT(Entry, ''%T'') AS ''Entry'',
    TIME_FORMAT(`Exit`, ''%T'') AS ''Exit'' FROM timemgt.daymaster
    WHERE TDate >=''' + @frmDate + ''' AND TDate <= '''+ @toDate +'''')

WHERE TDate >= ''''' + @frmDate + '''''

WHERE TDate >= ' + @frmDateString + '

declare @sql as nvarchar(max);
set @sql = 'select * from openquery(
SERVERNAME,
''
select etc
where SomeDateField > = ' + @dateString + '
etc
''
)
';

exec sp_executesql @sql;