There is no box 1b on 1099B form but Turbo tax insists on something being put there and it won't accept Blank

Blank is not a valid entry. Form 1099-B, box 1b is Date acquired. The IRS needs to know if the investment is short-term or long-term. This will affect how the investment is taxed. It may be taxed as ordinary income or taxed using capital gains rates.

• If the 1099-B form is blank but you know the date you purchased or acquired the investment, enter the date.
• The other option, if it's multiple dates, select the radio circle, Something other than a date and select Various.

Here are some other tips:

• If you held the security for more than 1 year, then enter a  "Date Acquired"  that is more than 1 year before the date you sold the security.  
• If you held the security for less than 1 year, then enter a  "Date Acquired"  that is less than a year before the date you sold the security (1/1/2015 will work).

 

If you aren't sure how long you held the security, try looking for the "Box" or "Code" on your 1099-B.

• If Box A or B is checked, then the holding period was less than one year. 
• If Box D or E is checked, then the holding period was more than one year. Source: RayW7

The letter on the Box is "X". According to the instructions on the back, Box 1b may be blank if Box 5 is checked (which it is "Non Covered Securities - Code X (unknown Holding Period") or if the securities sold were acquired on a variety of dates.