How do I target users before an upcoming date, like a renewal or trial expiry?

Store the date in a Date tag (for example, RenewalDate) and filter on it with the between N and M days ahead operator in the Segment Builder.

The operator matches users whose date falls in the next N to M days:

0 means today, 1 means tomorrow, and so on.
Both N and M are included. For example, RenewalDate between 0 and 30 days ahead matches renewals from today through 30 days from now.

Like the other relative date operators, it compares the date to today, so the segment updates automatically each day.

Other Date operators you can combine it with:

Past ranges: during the last N days, exactly N days ago, N or more days ago, between N and M days ago.
Yearly dates such as birthdays, ignoring the year: is today, is in N days, is N days past.
Fixed dates: on a specific date, on or after a specific date, on or before a specific date, between dates.