Excel handles time perfectly fine.
Do NOT graph date on Y axis and time on X axis. That makes no sense at all.
Put date/time info in one column in chronological order.
You will have two columns. One for date/time the other for the data corresponding to time.
Simply click the “create chart” icon on the toolbar, select both columns (the whole data set) to graph. Make sure the X-axis is time and the Y-axis is the data that changes with time. If the dates and times are evenly spaced, check to make sure that each tick corresponds specifically to the times that are part of your data set. Odds are, they are not. You can set the number of ticks within a range on an axis, and probably Excel has automatically created evenly spaced tick marks and your data doesn’t fall exactly on any of these.
If that doesn’t make sense and you’re still stuck, post the data here and I will make your graph or upload the excel file somewhere that I can access.
Pretty sure I can solve your problem in a minute or two.