Prior to v3.73, dp() (without an argument) returned Unix Timestamp. But with the new version, presumably because you changed the dp() code to accommodate other formats like ISO, dp() returns a "invalid arguments" error.
The workaround for this is to change all references to dp(a0) (that is, add 0 seconds to timestamp). But I thought I should report this so you're aware you created a breaking change without notice.
Regards