casper's postponing of snap refresh broken by switch to busybox date

Booting the latest focal live server daily gets us this:

Begin: Disabling snap refresh... ... date: unrecognized option '--iso-8601=seconds'
BusyBox v1.30.1 (Ubuntu 1:1.30.1-4ubuntu5) multi-call binary.

Usage: date [OPTIONS] [+FMT] [TIME]

Display time (using +FMT), or set time

 [-s,--set] TIME Set time to TIME
 -u,--utc Work in UTC (don't convert to local time)
 -R,--rfc-2822 Output RFC-2822 compliant date string
 -I[SPEC] Output ISO-8601 compliant date string
   SPEC='date' (default) for date only,
   'hours', 'minutes', or 'seconds' for date and
   time to the indicated precision
 -r,--reference FILE Display last modification time of FILE
 -d,--date TIME Display TIME, not 'now'
 -D FMT Use FMT for -d TIME conversion

Recognized TIME formats:
 hh:mm[:ss]
 [YYYY.]MM.DD-hh:mm[:ss]
 YYYY-MM-DD hh:mm[:ss]
 [[[[[YY]YY]MM]DD]hh]mm[.ss]
 'date TIME' form accepts MMDDhhmm[[YY]YY][.ss] instead
done.