SysTime.add!"days" missing

[Spacen Jasset]

I would like to subtract 60 days from a SysTime, but find that 
SysTime.add!"days" is not available, unlike Systime.roll!"days" 
which is available.

main.d(18): Error: template instance add!("days") add!("days") 
does not match te
mplate declaration add(string units)(long value, AllowDayOverflow 
allowOverflow
= AllowDayOverflow.yes) if (units == "years" || units == "months")


Why is "days" add missing? How might I get round this?

Regards,

Spacen.