To convert time to half hour interval, use the following formula:

Here, A1 is the cell containing the original time value.

Let me break down the formula for you:

1. HOUR(A1) extracts the hour value from the original time.
2. MINUTE(A1) extracts the minute value from the original time.
3. FLOOR(MINUTE(A1)/30,1)*30 rounds down the minute value to the nearest half hour. For example, if the original minute value is 16, this formula will round it down to 0. If the original minute value is 56, it will round it down to 30.
4. TIME(HOUR(A1),FLOOR(MINUTE(A1)/30,1)*30,0) combines the rounded down minute value with the original hour value to create the new time value.

To use this formula, simply replace A1 with the cell reference containing the original time value. Then copy the formula down to the rest of the cells in the column.