Add a Write To File Feature To My Existing Mac Os App

I have a medication tracking type app, that is very simple one button. When this button is pressed it grabs current time and the lets me do some calculations on it to show the last time I took the med via UserDefaults. It also shows the Duration of time from Current time to last time I took med in hours and minutes. Now all I need to implement is a Write to File feature when the button is clicked, I want it to store the current Date and Time to a Text File that is saved in the same directory as my App. It needs to not overwrite this Txt file every time something is saved, but rather it needs to log the current date and time on a new line. This way I can keep a nice log of the Date and Time I took each dose. That's it for now, however I want to add some other ideals I have to this app after this. So I need someone who is willing to help me on a day by day basis. I'll pay per feature added, or for the help of adding a feature. So you need to be able to have a good understanding of english and also be able and willing to listen to my feed back and my ideals and implement them the way I want. I have alot of work I need to get done in a short matter of time. I need other apps built and as I said before I need help implementing other Ideals I have from time to time. So I would like to form some type of trustworthy relationship with the FreeLancer so things run smoothly.