This is an interesting question, and some refer to the "morning" and "evening" sacrifices that were later instituted, as a specific time of day. I would look at this somewhat differently. The literal Hebrew could be:
(וַֽיְהִ֖י מִקֵּ֣ץ יָמִ֑ים)
"And it was in the end of days (or time) "
So connecting this idea i.e. "in the time of fulfillment," with the following phrase, "Cain brought the fruit of the ground," it would appear that the time of day is not at issue, but the time of year, the time of harvest. Able, also, brought the firstborn of the flock, during the fall, after the spring birthing of the flocks, assuming that the seasons followed as we understand them now. In my mind this would indicate that there was some knowledge of what God intended, therefore we find the difference between Cain and Abel's sacrifice. Abel obeyed God in His instruction. We know this from " "By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks." (Heb 11:4 ESV)
The lesson I would take away is that we must strive to understand God's design and will for us, by hearing His voice (via the scripture and the Holy Spirit), and beyond our own desires, follow Him in His instruction, if it makes sense to us, and especially if it does not.