icalendar-uid-format

icalendar-uid-format is a variable defined in `icalendar.el'.
Its value is

"emacs%t%c"

Documentation:
Format of unique ID code (UID) for each iCalendar object.
The following specifiers are available:
%c COUNTER, an integer value that is increased each time a uid is
generated. This may be necessary for systems which do not
provide time-resolution finer than a second.
%h HASH, a hash value of the diary entry,
%s DTSTART, the start date (excluding time) of the diary entry,
%t TIMESTAMP, a unique creation timestamp,
%u USERNAME, the variable `user-login-name'.

For example, a value of "%s_%h@mydomain.com" will generate a
UID code for each entry composed of the time of the event, a hash
code for the event, and your personal domain name.

You can customize this variable.