Yes! If you mark a challenge as "Done" more than 24 hours after it was assigned, you can still receive partial credit (60 points) for a late completion, but it will not count towards a streak or a pact.
To view past challenges, visit the front page of Daily Challenge and look for a link to the lower left of today's challenge that says, 'Past Challenges.' If you have trouble finding it, here's a shortcut: https://challenge.meyouhealth.com/past
On this page, you will see a long list of challenges in reverse-chronological order. The top challenge is from today, the next one down is from yesterday, and so on. Any challenge with a green check mark has already been marked as "Done." Any challenge you were assigned within the last 7 days but did not complete can be completed for late credit by clicking the challenge title and then the "Done" button.
Sharing a "How I Did It" post for a late completion will still earn you 50 points as well.