Sherlock

The Problem:
Sherlock Homes was listening to a strange tape that
had been left at the home of Lady Webberly: "At the
sound of the tone the time is 7:00 PM. exactly, Monday,
May 3,1987. In precisely 16,700 minutes you must be
under the oak tree behind the Somerset library, or you
will be in grave danger." Sherlock instructed Lady
Webberly to follow the instructions and that he would
follow behind at a discreet distance. On what day and
at what time was Lady Webberly to be under the oak
tree behind the library?