+a string. Anywhere a numeric value is required, an arithmetic expression
+may be used, provided it is surrounded by parentheses. Supported operators
+are:
+
+ addition (+)
+ subtraction (-)
+ multiplication (*)
+ division (/)
+ modulus (%)
+ exponentiation (^)
+
+For time values in expressions, units are microseconds by default. This is
+different than for time values not in expressions (not enclosed in
+parentheses). The following types are used: