In addition to the advice already given, I can deduce the following possibility from your question:
- You still want the result to be a date.
- But you want to 'discard' the Days, Hours, etc
- Leaving a year/month only date field.
DATEADD(MONTH, DATEDIFF(MONTH, 0, <dateField>), 0) AS [year_month_date_field]
This subtracts the amount of partial months from a base date (zero) and then adds them. Rounding down to the month that the date is in does as a result.
I hope this helps you.