Hey Jacquie,
I tried to replicate this and was unable to, the steps I took are as follows.
1, Select a Customer from the Customer List screen.
2, Click on the Activities tab.
3, Click Go To beside an existing activity.
4, Click on the field "Due Start Time" and selected a new month.
If the problem was that the pop up calendar was cut off, either resize the window or select the date field again, the popup will change locations if it cannot fit on the screen.