Hello all, NiFi Jolt question

Does anyone know if it's possible to convert timestamp formats using Jolt using 'field.value' the way you can using update Record? 

I'm aware that I can use an attribute value in Jolt to convert a field, like:

"date" : "${timestamp:toDate('yyyy-MM-dd HH:mm:ss'):toNumber()}"

but I was wondering if this function can be done using Jolt on incoming 'field.values', have tried using the default spec as below but it doesn't change the format.

Thanks in advance.

Andy