React: how to get date from AG Grid row data - reactjs

Question

Description: I'm trying to format the dateTime I get from an Activity data array. I'm certain the tabular data isn't empty or nullish since I've been using a different table library on which it displays.
Problem: The page displaying my activity list table crashes as it can not find a defined date in the data I've given it. This is likely due to me not guessing the data type of the valueGetter() when I define columns. The documentation and tutorial I've found were in javascript, so I couldn't get the answer from them.
Here's the ActivityModel interface:
export interface ActivityModel extends obj {
id: string
key: string
title: string
description: string
dateTime: Timestamp
duration: number
notes: string
type: string
activity: string
completed: boolean
material: string
size: string
assignedTo: string
value: string
unit: string
editable: boolean
}
And here's the Activities page:
const [columnDefs] = useState([
{ field: "title", headerName: "Title", filter: "agTextColumnFilter" },
{
field: "description",
headerName: "Description",
flex: 3,
filter: "agTextColumnFilter",
},
{
field: "startDate",
headerName: "Start Date",
flex: 2,
filter: "agDateColumnFilter",
valueGetter: (p: ActivityModel) => {
return formatDate(p.dateTime.toDate())
},
},
{
field: "completed",
headerName: "Completed",
filter: "agTextColumnFilter",
},
])
<div className="ag-theme-alpine" style={{ height: 400, width: 1200 }}>
<AgGridReact
defaultColDef={defaultColDef}
rowData={activities}
columnDefs={columnDefs}
onGridReady={pullActivities}
></AgGridReact>
</div>