Calendar (iCal) .ics format
The iCalendar specification outlines what is required in an .ics file.  In short, a simple .ics file looks like this:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//hacksw/handcal//NONSGML v1.0//EN
BEGIN:VEVENT
UID:19970610T172345Z-AF23B2@example.com
DTSTAMP:19970610T172345Z
DTSTART:19970714T170000Z
DTEND:19970715T040000Z
SUMMARY:Bastille Day Party
END:VEVENT
END:VCALENDAR


The UID is a unique identifier.  It's mostly used for sequencing multiple iCalendar requests.  The iCalendar specification gives guidance for how to create it:


  A good method to assure uniqueness is to put the domain name or a
  domain literal IP address of the host on which the identifier was
  created on the right-hand side of an "@", and on the left-hand side,
  put a combination of the current calendar date and time of day (i.e.,
  formatted in as a DATE-TIME value) along with some other currently
  unique (perhaps sequential) identifier available on the system (for
  example, a process id number).  Using a DATE-TIME value on the
  left-hand side and a domain name or domain literal on the right-hand
  side makes it possible to guarantee uniqueness since no two hosts
  should be using the same domain name or IP address at the same time. 
  Though other algorithms will work, it is RECOMMENDED that the
  right-hand side contain some domain identifier (either of the host
  itself or otherwise) such that the generator of the message identifier
  can guarantee the uniqueness of the left-hand side within the scope of
  that domain.


Most calendar applications are pretty forgiving of malformed .ics files.  If your events are pretty simple (no recurrence, no invitations, etc), it's probably not too difficult to knock together an AppleScript (or whatever scripting language you prefer) to create these for you.  Here's one AppleScript for it from StackOverflow.