The only reference I was able to find to this was questionable on how to actually calculate the 10 day prior period is on a somewhat questionable site IMHO and it states:
Luckily, there are not many variations of answers here and no need to debate. You can enter US only 10 days before the start date on your I-797 petition. i.e., say if your start date is October 1st, you can only enter US on or after September 20th on H1B visa. You can read the above VFS Agency FAQ, also below.
Personally I don't think that 20th will work but 21st should fall within the 10 day grace period.
However, you should be aware that granting you 10 day prior entry is entirely within the discretion of the CBP officer at the port of entry.
PORT OF ENTRY AND I-94 RETRIEVAL INFORMATION
You may enter the U.S. ten (10) days before the start date on your I-797 approval notice. At the port of entry, you may request that your I-94 card be issued with an additional ten (10) days beyond the expiration data of the I-797 approval notice. CBP officers at the port of entry are not required to grant these additional days, but you may still request them. If you receive additional days, you are not authorized to work beyond the expiration date of the I-797 approval notice.