You can use basic JavaScript scripts in ${...} syntax to calculate variables.
For example, to format a timestamp in different ways. Digdag bundles Moment.js for time calculation.
timezone: America/Los_Angeles
+format_session_time:
echo>: '${moment(session_time).format("YYYY-MM-DD HH:mm:ss Z")}'
+format_in_utc:
echo>: '${moment(session_time).utc().format("YYYY-MM-DD HH:mm:ss")}'
+format_tomorrow:
echo>: '${moment(session_time).add(1, ''days'').format("LLL")}'
+get_execution_time:
echo>: '${moment().format("YYYY-MM-DD HH:mm:ss Z")}'