"The angels and the Spirit ascend to Him in a day whose span is fifty thousand years." Qur'an 70:4
Allah (SWT) is beyond time and space as He created every thing including time and space. The verse is mentioning the period of one day which has specific importance and not like our usual days. It is for angles and it is equal to our fifty thousand years. This day is created by Allah and never rule over Allah (SWT).
There are many days mentioned in Quran with different periods. Day in Quran does not always mean the usual day in our understanding which is 24 hours, but it is a time unit, with different periods depending on the nature of the Day.
Allah (SWT) is beyond any timing, though He created the time foe us and ordered us to act according to these timings.