The timer for the build time resets sometimes and therefore it takes more time than usual to build headquarters. It has happened many times before and the construction took an extra day sometimes. I don't know if its a visual bug or a genuine one because sometimes the timer corrects itself and sometimes not and it takes a day longer.
I kept it to build yesterday.
And this is the timer from just a while ago.
The timer is now currently showing the right time after many re-login attempts.