Calendar 1.5.8 added support for calculating the date. Examples:
{{yesterday}}
{{tomorrow}}
{{date-1d:YYYY-MM-DD}} // yesterday
{{date+2d:YYYY-MM-DD}} // in two days
{{date+1M:YYYY-MM-DD}} // next month
{{date+10y:YYYY-MM-DD}} // in 10 years
{{date +7d}} // use default format