Counting the number of unique values by date in R

Please help me to count the number of unique IDs per Date. so, initially, there is this data frame of IDs and dates

 ID         Date 
 1        2009/11/1
 1        2009/11/2
 1        2009/11/2
 2        2009/11/1
 2        2009/11/1
 2        2009/11/2 
 3        2009/11/1
 3        2009/11/3  

It is possible to rearrange it by date. If we do so then we will see that on the 1st there are 3 unique IDs. On the 2ed 2 unique ID and on the 3rd there is one unique ID. So the final table should look like this:

  Date      uniqueIDs
2009/11/1      3
2009/11/2      2
2009/11/3      1

I know that it is possible to aggregate with aggregate by using sum if the value is '1' or '0 'like that:

aggregate(DataFrame$RoomAv ~ DataFrame$Date, DataFrame, sum)

But how to count the unique number of IDs per day? The ID column is an integer column.

Thanks a lot!