Tying to calculate the sum of days from two date time inputs?

[DangerDrome]

Tying to calculate the sum of days from two date time inputs, such as:

{{ DAY(SUBTRACT(end, start)) }}
or
{{ SUBTRACT(DAY(end), DAY(start)) }}

But just getting 0, debug doesn't seem to display anything:

Probably user error...

[DangerDrome]

After some breif debugging, the following don't appear to be working?

YEAR(a) | get year of a date object, similar to getFullYear
MONTH(a) | get month of a date object, similar to getMonth
GET_DATE(a) | get date of a date object, similar to getDate
DAY(a) | get day of a date object, similar to getDay

However this seems to work:

{{ SUBTRACT(GET_DATE(DATE(end)), GET_DATE(DATE(start))) }}