Whether you can enter jhāna or not, when you sit down to meditate, you can make a prior resolve to do something when you have finished your meditation session. Then you do not have to think about it while meditating. It is similar to remembering before going to sleep, “Tomorrow is the rubbish collection day. I must remember to put out the trash in the morning.” Then you can sleep comfortably, without thinking about it again until you wake up, when you will immediately remember what you have to do today.
If you practice meditation regularly for a specific time, say half an hour or one hour, you will find that you have no need of an alarm timer. With regular practice, you will know instinctively when the time is up.