I agree with giving them a visual. Make a calendar of the week. And make the boxes big enough for several pictures. Show the entire week on the calendar:
Sunday (3 meals, bed for sleeping at night), Monday (Daddy in a handdrawn car or on a cutout pic of a plane, - however he left for out of town, dinner, bed for sleeping at night), Tues ( 3 meals, with a pic of a telephone where they talked to Daddy, bed for sleeping at night), Wed (3 meals and add whatever activities you have planned - the library? pool?, and a bed for sleeping at night) etc... all the way to Friday... where you show breakfast (picture of pancakes?), lunch, an activity, and a picture of Daddy with your house (draw the house if you need to, a box with a roof will work!).
Then let them mark off the days (and the parts of the day if they want) until Daddy is back home. They will be able to "see" the time. It's more tangible. They'll know they have __ many bedtimes, and _ many breakfasts and lunches, and a trip to the library and a trip to the grocery store, etc until Daddy will be back home. You could even spend some of the time distracting them with plans for a special dinner when he gets home. And making a welcome home banner.