The "date" (or "epoch") is actually just a huge number - the number of seconds since 00:00:00 UTC, Thursday, 1 January 1970. Going a little past midnight on 12/31/2036 would require an additional digit (in hex), and there may not be space for it in that app (the variable for the epoch may be only 32 bits). The same thing happens for dates much earlier than midnight on 1/1/1902 (more than 32 bits with a minus sign in front). Some calendars may allocate a larger variable for the epoch.
(You're too young to have been there, but read up on the Y2K panic. "No one will ever need more than 2 digits for the year, so we can save expensive storage space." [5MB - not GB - hard drives were VERY expensive when hard drives first came out. Saving ] Then came 2000 and everyone realized that in a few years you wouldn't be able to tell by the birthdate if someone was 2 years old or 102 years old. Not enough digits allocated. Same problem with the epoch - if it's only 32 bits long, it's limited to 1902 to 2036.)