How do I only listen for datetimepicker valuechanged event from user input

I use a Boolean "flag" to control this.

A private property on the form e.g.

bool codeUpdate;

When I need to programmatically change something I first set the boolean to true, do my changes, then set it back to false e.g.

codeUpdate = true;
//make my changes
// …
codeUpdate = false;

Then in each of the control's events that I want to not fire if I'm doing it in code I have a single line

if (codeUpdate) return;

This is very old-school by the way!
A more modern approach might be to actually unhook the event(s) entirely while you make your programmatic changes e.g.

DatePicker1.ValueChanged -= DatePicker1_ValueChanged);
// make my changes
// …
DatePicker1.ValueChanged += DatePicker1_ValueChanged;

I prefer the first approach - to me it's clearer what's going on, especially if you use an appropriately named variable. I'm sure someone will put forward an alternative argument though.

To be honest, I probably wouldn't keep changing the Max Date through the Enter event. I would just validate the DatePicker value against "yesterday" in the ValueChanged event then this problem just goes away