You really shouldn't store calculated values.
IN a query, you find the last record entry related to the parent record and then in a calculated field return that date.
I've said that very simplistically because we would need to know a little more about your data to ensure that you are looking at the newest entry vs. another entry.
HOWEVER, in a single Form\SubForm relationship, in the code for the subform you can refer to the Main form by using the "Parent" just like "Me." or "Me!"; thus, referring to the parent.
So, if the control on your parent form is named "followupdate"
then you can refer to it by
parent.followupdate = yourvalue
, be aware that you have to use the proper text.
Look at the detail section of the subform...
NOW BE CAREFULL HERE: When
Referring to Items on a Sub-Form
there is a CONTROL that acts as a CONTAINER for the actual form and it is the detail section of the actual form that you need to look at, I suspect that you were looking at the container.
BTW:
I will use this reference when I get lost trying to refer to controls within forms or from other forms:
Forms: Refer to Form and Subform properties and controls
I'm worried enough that this site might ever go down that I took advantage of the download offer of a hardcopy of the table - upper right hand corner!