The Y2K problem came about because programmers tried to write really tight programs to save expensive memory and storage space. In their code, they just used the last two digits of the year. This means that when Y2K arrives, the software will switch to 00. That will be interpreted as 1900, or as the date the hardware was initialized. Modern hardware and software products use a four-digit number for the date, but some of the applications you use or the firmware in some of your hardware might have been written before the millennium issue became a consideration.
Many newer applications are just older programs that have been added to and updated.
Similarly, some of the hardware you might be using could be limited by its firmware. Another little-known component of the Y2K scenario issue is that 2000 is a leap year. Thus, even if date routines use a four-position year, there is a concern that they may not incorporate the necessary logic to recognize that 2000 is a leap year. A leap year only adds a day to the calendar, so if this function fails, it should be more of a nuisance than a disaster. If not, you might have some software patching to do.