Cancellation Anomaly

I had to cancel a reservation made for the 24th March. At 11.30 on the 21st I logged on and made the cancellation ie 72 hours + 30 minutes before arrival time. I have just looked at the cancellation email timed at 18.51 on the 21st which states I cancelled within the 72 hour notice period which is incorrect. Has anyone else ever seen this happen? and is there any way I can prove the time I cancelled? I will contact HQ on Tuesday and explain the situation.