Crystal Reports group conditions in count function

Message

   Hi,
Im using the count function and im trying to group by time. Based on the crystal reports help, the format should be Count(fld, condfld, cond)

So, following this format, I created a formula field where I inputed:

count(some_field, start_time, 'for each minute')

However, when I do this, I am informed that "This group condition is not known". This comes as a surprise because the crystal reports documentation says that 'for each minute' is correct.

So, my question is this: how do I do the above count and group by minute?

Thanks in advance. 