This article explains how to use handlebars helpers to calculate the number of days between dates.
21*24*60*60 = 1814400 = seconds in 21 days.
timestamp:
{{timeStamp}}
date+offset:
{{dateAdd myDate 1814400000}} //dateAdd uses milliseconds
timeStamp in seconds from epoch:
{{dateFormat 'X' timeStamp}}
myDate in seconds from epoch:
{{dateFormat 'X' myDate}}
myDate in seconds + 21 days:
{{add 1814400 (dateFormat 'X' myDate)}}
{{#compare (add 1814400 (dateFormat 'X' timeStamp)) ">" (dateFormat 'X' myDate)}}
21 days from now is greater than myDate
{{else}}
21 days from now is less than myDate
{{/compare}}