The Retentive property of an Internal Tag is not working, because it does not save the last value when stopping a Domain. What can be wrong?

The Retentive property is not aimed to do this. It is only useful in Hot-Standby applications, to keep an Internal Tag value when there is a server switching.

How to save the value of an Internal Tag when stopping a Domain?

By using Data Server's Save method.

How do I display the current system date and time on a Screen in my application?

Create a CurrentTime-type Demo Tag and link it to a Display on this Screen. A date and time format can be configured on the Formatting tab of a Display properties window.

How do I create a timer in Elipse E3?

By using a Square-type Demo Tag. This Tag must have its Enabled property set to False and its Period property set to twice the limit time, in milliseconds. By setting True to its Enabled property starts counting time. To execute any script at the end of this timer, create an event linked to this Demo Tag's Value property, which is executed when that property changes its value. In this script, users must also disable that Demo Tag, that is, set its Enabled property to False, so that it stops changing. It is recommended to use Demo Tag's Reset property before setting its Enabled property to True.