I have a question regarding the Police clearance certificate requirement for express entry. I obtained my PCC around 1 month before leaving the country (South Korea) for good. Is this police clearance valid? since I have no person currently in South Korea who can obtain the PCC for me.
Do they check exact dates I left the country ?
For any country which is not your current country of residence, you always need to obtain the PCC AFTER you've left the country for the last time. Current rule has changed a little bit and say that you need to obtain it after your last stay in that country which was over 6 months in a row, but it is always safer to get it after you finally leave the country.
In your case, if your last stay in South Korea was for more than 6 months, it will cause rejection surely. Even if it was for lesser duration, there's a high chance of rejection. Incorrect PCC is one of the foremost reasons of rejection.