Visa Expiry Date

Hello,
I have been given my passport with a visa yesterday. My problem is that even though I have been granted a visa for 2 years and 3 months, the visa expiry date calculates to 3 months only. (Date of Issue- Sept 2019 and Date of Expiry - Dec 2019). Shouldn't the expiry date be Dec 2021?
Please reply someone, I am going to attend a university which starts soon.

The expiry date of the visa is not the same thing as the expiry date of the period of stay which will (presumably) be granted upon entry.

You have a visa with which you can gstay inh Japan for 2 years and 3 months (I assume this is a student visa).

And that visa is valid for 3 months, until Dec. 2019, meaning you need to genterh Japan before it expires.

So if you travel to Japan between Sept. and Dec. of 2019, you can stay for 2 years & 3 months after you enter Japan.

Visa and resident status.

Visa sticker on your passport.
Expiry date, mean you must enter Japan by the expiry date.
Period of stays, is the length of stay given to your resident status in Japan,
normally that date will start from your entry date.

Once you enter Japan, your visa is cancelled(unless it is multiple-entry, another story) and you will be given a resident status/zairyu card with length of stay stated on your visa. The period/length of stay date start from your entry date and expired 2 years 3 months later.