Let's say 7:15, in front of the theater.
(What is the use of 'Let's say'? Can we say 'Let us say' instead ? When do we use 'Let's say'?))
The use of "Let's say . . ." is to set an approximate time to meet. It implies uncertainty -- but within a few minutes either way.
Since this is informal speech, we wouldn't use "Let us say . . ."