WHERE date_col1 >= DATEADD(DAY, DATEDIFF(DAY, 0, date_col2), 0)

AND date_col1 < DATEADD(DAY, DATEDIFF(DAY, 0, date_col2) + 1, 0)

Or:

WHERE date_col1 BETWEEN DATEADD(DAY, DATEDIFF(DAY, 0, date_col2), 0) AND DATEADD(MS, -3, DATEADD(DAY, DATEDIFF(DAY, 0, date_col2) + 1, 0))