Hi hanistar18
Thank you very much for your question.
Yes exactly: it means you have time from Monday 25th of July at 00:00 until Tuesday 26th of July at 05:00 am to make this journey.
If you only go to Grindelwald one day (on Monday) and come back the same evening, it is alright.
But, if you go to Grindelwald on Monday and plan to stay there a couple of days, it is better to buy 2 separate tickets: one for Monday (one way Zürich > Grindelwald) and one for the other date (one way Grindelwald > Zürich).
If you have any more question, please call our colleagues at the SBB Contact Center (0041 848 44 66 88, CHF 0.08/min), they are available 24/7.
I wish you a great time in Switzerland.
Greetings
EleonoraS