Can I execute multiple sequential commands in a cron using a semi-colon?
 

Is that how it works?

Example

Every 15 minutes

1. print the date and pipe output to create/overwrite log.file
2. update apt and pipe append output to log.file
3. print the date and pipe append output to log.file (this lets me know how long the apt update command took to finish

Yes, that should work but it seems silly to run an update every 15 minutes. I guess it depends on what you are actually trying to accomplish. Since the file is being overwritten every time you can not determine if it changes. You can also write the output of the time command to a file to see how long apt takes to update i..e

time apt update > log.file

did you try it already?
I would recommend you to write a shell script and put everything into it.

Oh yes, the actual command rysnc's a very large directory over the network. I am trying to figure out roughly how long this rsync takes.

I only ever want to know how long and when the last rsync occurred.

Duh, of course lol. I'm hesitant to dive into the rabbit hole of shell scripting but I guess something like this is actually pretty simple isn't it.

time command is your friend.

The last crontab field accepts a complete standard shell (no bashism), except that a literal % must be escaped \%

The shell allows to group commands for a common redirection:
The following allows the apt to run up to twice the interval time (2*15=30 minutes):