Should you propose a backup plan if she cancels the first date?

The message she sent is the best evidence you have. A cancel that names a real thing, like a shift change, a sick family member, or a cancelled train, is usually sincere. A cancel that says "so sorry, rain check?" without a reason and without a day attached is more ambiguous. Look at whether she apologised in a way that sounds like she wanted to go, and whether she moved toward a new time herself or left it open. If she offered a new window, the interest is still there and you can simply pick one. If she left it open-ended, the more useful move is to find out whether the open end is real.

Also look at timing. A cancel an hour before the date is different from one three days out. A last-minute cancel is more likely a real emergency, and it tells you little about her general reliability. A cancel that comes after several rescheduling rounds is a pattern, and one backup offer shouldn't be your whole response to it.

When you do propose a replacement, make it a single day and time, or two at most, and tie it to something she can say yes to in one word. A menu makes her do the work of narrowing it down, and it can read as though you're fishing for the most convenient slot. One clear option that's easy to accept keeps the thread moving. Name the same kind of plan she already agreed to, so she isn't weighing a new idea and an old one at once.

If she replies with a yes, lock it in right away with the day, time and place. Don't leave the details floating, because a half-confirmed plan is how a second cancellation happens. If she says the new time doesn't work, don't argue for it. Ask what would, and take the answer as information about her availability.

If you've made one clear offer and she hasn't taken it, the next message should not be another offer. Let the silence or the vague answer stand, and notice how you feel about that. Chasing a second and third time usually reads as anxious, and it tends to turn a mildly interested person into someone who is now reluctant to reply at all. You aren't entitled to a date, and you don't need to win the argument about whether she's serious.

Give her a few days and let the thread breathe. If she comes back on her own with a new suggestion, that's the strongest signal you'll get. If she doesn't, close it out lightly with a friendly message that leaves the door open without asking again, then move on rather than waiting by the phone.