I guess that the NOW() function creates the current date and time,

Yes it does, and the

-"00:01:00"

part subtracts one minute from the current date and time, thereby returning the time value for one minute in the past.

but I don't see why this is nescesary when I want to count the observations 60 seconds back in time from the timestamp in ex column B?

If you didn't have that, how do you expect Excel to know to what it should compare the timestamp in column B? i.e. Column B contains 12:30:46 and you want Excel to know whether to count this cell...so you must somehow tell it what you are comparing it

*to.*
When you write B1:B10 in the SUMPRODUCT function - should I place this in the 10th row

No, you put the formula wherever you want the counted result to appear. The B1:B10 refers to the range containing all the timestamp values - you just adjust it to suit. I think you said you have ~65000 values, so maybe rather than a range, just specify the whole column using B:B (but don't expect lightning-fast calculation time!)

and how can I make it dynamic so that it will always count all the obervations 60 seconds back in time from the timestamp?

The formula is dynamic in the sense that, every time the worksheet is calculated, it will count how many timestamp values are greater than the value in C1, and the value in C1 will always equal the time it was one minute ago.

And just because I'm new with this I would like to know what the "--" means in the SUMPRODUCT function?

It converts TRUE values to 1 and FALSE values to 0, all of which are then summed to give you a total of the number of times the condition is TRUE. The condition is the

B1:B10>=$C$1

part.

So if 3 values within B1:B10 are greater than or equal to the value in C1, the formula counts 3 TRUE values, which is 3 * 1 = 3.

And also what the $ signs mean when you write $C$1?

It locks the reference to C1, so that wherever you copy your formula it will always refer to C1.

Have you actually tried implementing the formulas with unexpected results, or are you pre-judging the validity of it?