Custom form variable date format

I am trying to set up a canned response using variables defined in a custom form. For example:

[custom form definition] "Serial number", short answer, variable name=serial_number

In the canned response I can use Serial number: %{ticket.serial_number} and it works fine. However when I try to do the same for a custom date field, eg

[custom form def] "Received date", date-time picker, variable=received_date

using Received date: %{ticket.received_date} or %{ticket.received_date.short} always returns 01/01/70 as the date. Am I doing something wrong or are custom date variables not supported?

Thanks.