Setting Up One That Picks Up An Entry In One Data Table And Then Sums A Field?

Im pretty use to doing loops (although still relative new to vb) in my code however am having difficulty setting up one that picks up an entry in one data table and then sums a field in another where records have matching criteria. Example:

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What i am trying to do is set up a datagridview on a form that shows the summary data. The time is controlled on the form by a trackbar (output to textbox) and I started setting up a loop using (For/Next) to isolate firstly the matching entries in field 3 (time). This works fine. I then used compute to sum the the field 2. However what i want to do is to loop through table 2 and where entries match i.e. where Field 1 in table 2="A" sum matching entries in table 1 where the DateTime matches.