textbox1 textbox2 textbox3
7:42 am 12:16pm 04:34

On textbox3.refresh
this.Value = ttoc(difftextbox1-2,2)

textbox4 textbox5 textbox6
7:42 am 12:16pm 04:34
On textbox6.refresh
this.Value = ttoc(...,2)

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textbox12 sum(09:08 ) .
On textbox12.refresh
compute sum of textBox3+textBox6+...
this.Value = ttoc(sumValue,2)

on every Valid event for textBox1,textbox2...
thisForm.Refresh