I would post my question to the Salesforce community but I am unable to login there for the last couple of days for an unknown reason.
Today I create to near-identical custom fields on the contact object in my sandbox. One was a datetime field and the other a time field. Both have the same security and view/edit permissions.
The first immediately became visible to the process builder to be used in a field update action. The second just doesn't appear. Typing in the entry box doesn't bring it up either.
I've confirmed I have picked the correct object to update, as I have already selected other contact object fields to be updated in the same workflow.
Any help greatly appreciated.