How to pass global string var to a set_datetime service in NR?

I try to create a new value in a datetime helper by a global var.
I don’t get it working.
It gives an API error that at least one date time stamp
The global.sproeier_aan is HH:mm format

If a do a hard figure like:

It is working, but with global var not

I did found the solution:
just put the var between quote’s: