The incorrect utilization of lags is considered as the principal reason for generated errors when using multi-calendars. It is important to distinguish between two types of lags:

https://mosaicprojects.com.au/PDF/Chronographic_diagramming_method.pdf

• 1. Lags that presents some technical constraints, such as concrete curing, and always use a continuous working period calendar. These lags should not be affected by non-working days.
• 2. Lags used as a replacement for an imposed quantity of workload, before the start or the finish of the successors activities. These lags are actually simulated with partial start or finish dependencies. These simulations are insufficient to ensure an accurate calculation of the schedule and could possibly generate wrong answers.

Spider Project solves this issue by providing time lag, volume lag and double lags.  

• 1. Time lag along with individual lag calendars solves the issue of continuous working periods. 
• 2. Volume lag solves the issue of an imposed quantity of work.  There is no need to simulate volume lag with partial start or finish dependencies.
• 3. Start-Start plus Finish-Finish with lags usually is sufficient but if activity execution is non-linear (due to variable productivity and different non-working time of connected activities) intermediate double lags solve potential problems.  
• 4. Double links solve the issue of partial start or finish dependencies.  The partial start or finish dependencies can be of the type Time Lag or Volume Lag.

I agree with the previouse comments, change the policy. If you are using P6 then allow the contractor to create a Project Calendar. Another option is to create and assign a rresource with a possesion calendar.

Regards

Paul E Harris

 You need to talk to whom ever created the policy of not being able to use custom calendars and find out what the reasoning is. This is one of the benefits of using a scheduling program is that you CAN define your working calendars and have multiple calendars. I don't understand why you think that creating a load of milestones can solve your problem the issue is remains the same whether there are milestones or not. You might as well as use a calendar hanging on a wall and put X marks on the day that you can work. If you can not plan your schedule to shows the planned working days your schedule will never be correct. You may as well NOT track the civil work and just add milestones but the problem is that there will be NOTHING driving those milestones. If the civil work is actually on the critical path and you can not accurately reflect that in your plan then it will never show critical.

Hi Philip

Any planning software worthy of the name can create such a calendar. (Although P6 may struggle)

The contractor had better change his policy amd his software.

Best regards