calculate the hours between 2 time fields

I am trying to calculate the hours between 2 time fields, please see below.

I have been using the following javascript and it's not calculating - can anyone please provide any assistance, or a different fomula

TypeScript

const startDate = moment(data.startTime)
const endDate = moment(data.endTime)

value = startDate.diff(endDate, 'hour')
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