Hi guys! I have read all of the comments below and I have some thoughts about dates:
1) We have three "old" dates: 03/20/86 (the inscription on the tape, you can find it above on this page); 4/5/1987 1:56am (from metadata of the savefile) and... 12/12/1987 - the log file with <o> tag has this date in it's name. I counted the number of days between these dates (without including the end date) and as a result I got the following: 381 days or 1 year, 0 months, 16 days (first and second dates), 251 days or 0 year, 8 months, 7 days (second and third dates), 632 days or 1 year, 8 months, 22 days (first and third dates).
Unfortunately there are no any visible sequences or coincidences in this numbers so I can't argue that these numbers make sense.
2) The one "modern" date: 11/15/19. The simplest assumption: something will happen on 15th of November. Update of the game? New hint from developers?