Making date.parse work for user's time zone

There is a nice formula in SuiteAnswers which provides the number of days between two dates.  The problem for me is that in the user interface we are entering dates in DD/MM/YYYY format and the formula assumes all dates are MM/DD/YYYY.

Here is the formula that I am using to set a field via Workflow.

(Date.parse({custbody_billing_period_to.id})-Date.parse({custbody_billing_period_from.id}))/86400000+1

So If I enter the following dates

5/4/2020 - 5/4/2020 that returns 1

increment by one Australian day

5/4/2020 - 6/4/2020 that returns 32 days

5 April - 6 April should be 1

it is interpreted as 4 May - 4 June= 32