When is best time to stretch am or pm?

That is totally up to you. However, science has proven that starting your day with exercise will make you feel good, energize you and give you a sense of accomplishment. It also won’t clutter your mind by thinking about it all day ‘I need to exercise’
Personally, I am not working right now and like to start my day by stretching for about 10 min. As soon as I open my eyes. Meaning, before I do anything other than a drink of water. I exercise mid day for about 30-60min. It gives me the energy to get through the afternoon and motivates me to make a healthy dinner. And I eat a bit earlier and was able to shed a bit of weight. It is working fabulous for me.
When I was working I would hit the gym before work at 6:00am and/or go for a minimum 40km bike ride after dinner.
I hope that helps.

Both are, if those are small 1 min stretchings: to wake up, to freshen up during the day, if you want to sleep well (right before bed time).
If you want to get better flexibility and do a 20 min stretching – than am.

It is best to stretch in the morning because you need to wake your muscles up and bring energy into your core . This makes you be open to intentions to set for the day. Wakes you up in general and leaves you generally more postive and able to cope with work tasks or general tasks though out the day 😊

The evening is better for a stretch for me personally. I usually have things I'd like to accomplish as soon as I wake up, but in the evening I'm in my room at my desk. So it's a nice reminder to get up, step away from the computer and do something good for my body