To calculate a date that is 60 days from a start date, including weekends, use the following formula:

Here's how it works:

1. START_DATE is the date you want to start from.
2. INT((60+WEEKDAY(START_DATE))/7)*2 calculates the number of weekends (2 days) between the start date and the end date.
3. IF(WEEKDAY(START_DATE)=7,1,0) adds 1 day if the start date is a Sunday.
4. IF(WEEKDAY(START_DATE+60)=7,1,0) adds 1 day if the end date is a Sunday.

You can adjust the number 60 in the formula to calculate different numbers of days from the start date.

For example, to calculate 30 days from the start date, you would use:

To calculate 90 days from the start date, you would use:

And so on.