To post time transactions:
Go to Transactions > Financial > Post Time.

If you want to update the Project Cost Variance Account list, select a different account from the list. By default, it is populated with the selected default project cost variance account. See "Setting the Default Project Cost Variance Account" for more.

In the Posting Period field, select an open accounting period.

NetSuite dates each created journal entry with the date of posting. Depending on your company's accounting settings, it may be possible to post time for transactions outside the selected period.

Note If your company uses the Require Approvals on Journal Entries preference, the Posting Period field is not available.

In regards to the last line there...why? I would want the posting period to reflect the date of the time sheet, not when it was posted.

have the timesheet approved while the period is Open. If the accounting period is closed by the time of approval NS will have to post it to the first open period after the date.

What’s happening now is my time sheet is dated dec 31 but it gets posted to Jan if I post it today

is the period open?

Yes