Mindfulness is about keeping your awareness in the current moment most of the time. It is about being present and aware in your life today. Mindfulness would say the past is the past and the future isn't here yet.
What has happened in the past has already happened. It cant be changes. It is possible to learn from the past, reflect on the past, or just have a memory of the past.
since the future hasn't happened yet, there is no point in excessively worrying about what is to come. And yet, it is effective to plan and prepare for the future.
Being in the past and future mindfully or intentionally is optimal. If instead you live your life mindlessly in the past or future , you can miss out on really important things going on right now and end up regretting it later.
Mindfulness would say: If you are going to to spend time and energy focusing on the past and future; do it intentionally and with full awareness. However, spend most of your time in the present moment, in the here and now, in today. This allows you to fully experience your life as it happening, rather than what it was or what it might be.