One of the features of Intercom is the ability to distinguish new users from old ones. To do this, Intercom needs to know when your existing users signed up. This date is sent to Intercom in the form of a Unix timestamp. Every modern programming language has ways to calculate these timestamps. Intercom only accept the timestamp in seconds not in milliseconds. Intercom will also accept ISO 8601 timestamps if generating a Unix one is troublesome.
What if I don't know or can't generate the timestamp?
You can also send Intercom dates in DD/MM/YYYY format. Then options such as “Message users who signed up in the past 30 days" will work correctly.