Where this is in the timeline is a mystery, atm. Tilly's an Ensign, and Michael is nowhere to be seen (does she have her own quarters now? Tilly has a second bed still). Time will tell whether there's room for this adventure before they pick up Pike, or even if Pike is commanding the ship during this short trek.
My guess is "during the S1 finale" - after the Paris ceremony, when the ship is back in space for routine missions, but before the teaser where they got the call from the Enterprise.
IMO the Federation isn't at war at the moment of this short - and apparently Tilly is an ensign now (I'm bad at spotting ranks), which she wasn't during S1. But the Discovery doesn't seem to be on a critical mission to explore the galaxy spanning mystery of red sparks either. It seemed like very day-to-day business.
Michael is nowhere to be seen (does she have her own quarters now? Tilly has a second bed still).
At the time of these events, Michael Burnham is somewhere staring at a wall and having heavy-handed internal monologues about the burden of responsibility
