On those days you don’t have as much time, you should still fit in a workout. You could do weights on one body part. Or you could do some crunches and push-ups. Another good one is to do intervals on the bike or treadmill, just do 1 minute regular intensity, 1 minute high intensity for 20 minutes. Then finish up with some planks or wall sits. There are tons of ways to get in a workout.
Not only can you get some physical benefits out of any exercise, but also you will be glad you are keeping yourself accountable and in the habit of working out. Living a healthy lifestyle means making your workouts a priority. Just because you cannot do it perfectly does not mean you cannot make some progress.