TD Ameritrade has been rejecting my calendar orders this year ("strategy is not being accepted"), so I devised a strategy which I call a "synthetic calendar" to get the calendar entered. It consists of a credit spread with the short strike at the desired calendar short strike and expiration, matched with a debit spread with the long strike at the desired calendar long strike and expiration. In essence, offsetting credit and debit spreads using the same strikes but different DTE. After the two spread orders have been filled, close out the long calendar spread, leaving only the short calendar in place. The double spread trade will initially have a very low delta and vega, and virtually zero gamma, so there is very small price movement or volatility risk, which means you can wait a day to close the long calendar to avoid the PDT rule. The obvious penalty for using this workaround is the extra commissions and fees. See the attached for an example SPX put calendar entry.