To answer this question, let us first of all look at some definitions.
In a previous Q&A we wrote:
“…the Great Tribulation (Matthew 24:21) is followed by the Heavenly Signs (Revelation 6:12-14; Joel 2:30-31; Matthew 24:29) and by the Day of the Lord (Revelation 6:17). The indication is, from Scripture, that the Day of the Lord begins one year before Christ’s return (compare, for example, Isaiah 34:8). But even as the Day of the Lord does not end with Christ’s return, but is ongoing, so the Great Tribulation (which begins approximately 2½ years prior to the Day of the Lord) does not end, either, when the Day of the Lord starts. Rather, it continues until Satan, who causes the Great Tribulation to occur, is removed from his throne.
“Christ returns to cut the Great Tribulation short (Matthew 24:21-22). This means, the Great Tribulation is still ongoing — it overlaps for one year, in that sense, with the Day of the Lord. So, all in all, the Great Tribulation lasts approximately 3½ years, but during the last year of the Great Tribulation, God starts to intervene — approximately one year before Christ’s actual return…” We also understand that God could shorten these days…