create a code on your checkbox click event:
private sub checkbox_click()
if checkbox then
if Trim(me.textboxDate1 & "") <> "" And Trim(me.textboxDate2) <> "" then
me.textboxToShowDiff = DateDiff("d", me.textboxDate1, me.textBoxDate2)
end if
else
me.textboxToShowDiff = ""
end if
end sub
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template code. supply correct textboxes, checkbox name to the code.