The verb diistēmi means to stand apart or at a distance — whether of spatial separation or the passage of time. In Luke's Gospel, it marks the passing of time in the passion narrative (22:59 — 'about an hour later') and at the Ascension (24:51 — 'while he was blessing them, he left them'). The same root appears in Acts 27:28 for measuring depth.
The two key Lukan uses of diistēmi frame the opposite ends of Jesus's earthly ministry's climax. In the courtyard of denial, time's passage is marked as Peter fails — 'about an hour later.' At the Ascension, Jesus departs while blessing His disciples. The same word that measures the interval of human failure also measures the moment of divine commissioning. The God who knows the exact intervals of our failing also precisely times our sending.