User is trying to create invoices with the As of Last Month date filter in the Invoice Filters and Options dialog to include any unbilled time from previous billing periods. However, this sets the Billing From date on the invoices to 1/1/1970.
This happens because Core uses a date range of 1/1/1970 (which is a set value) to the last date of the previous month when you use the As of Last Month filter. If you do not want this date to show up on the invoices, you should use a custom date range (Filter By > Custom). This lets you control the From and To dates precisely. Check Core Help Center for details on creating batch invoices.
Another way of handling this is to select the option Show billing through on invoices in Global Settings > Billing > Invoices. This option lets you show a Billing Through date (which references the end date of the filter) on the invoices instead of a From/To range. In this case, you can keep the filter As of Last Month so that the invoices show the last day of the previous month as the Billing Through date. Check out this article for details on changing the date range on invoices.