Hi Team, 

I have a SharePoint calendar used for equipment booking, I have created a responsive Nintex form and will like to restrict the start time to 830am or 145pm for the start time, I am not sure which method to use to set the two default values? I am trying out the regular expression ^(^0-9]|0|0-9]|1|0-9]|2|0-3]):)0-4]40-5]s(AM|PM)$ but ended in SharePoint error

There is two booking windows as such:

The start date also must start from 15th to 30/31th of the month if the current date falls within the 1st to the 14th of the month.

The start date also must start from 1st to 14th of the next month if the current date falls within the 15th to the 30/31th of the month.

Appreciate your expert advice.