To reason about dates and times in a purely declarative way with Prolog, the best way I have seen is library(julian) by Michael Hendricks: http://mndrix.github

To reason about dates and times in a purely declarative way with Prolog, the best way I have seen is library(julian) by Michael Hendricks: http://mndrix.github.io/julian/ http://mndrix.github.io/julian/ For example: ?- form_time([2014-5-1], T). T = datetime(56778, _5804), _5804 in 0..86399999999999. This uses CLP(FD) constraints to reason about time in a very general way, usable in all direction. For example, you can express "all times after 2014-5-1" as follows: ?- form_time([after(2014-5-1)], T). T = datetime(_1304, _1306), _1304 in 56778..514671, _1346#=86400000000000*_1304+_1306, _1346 in 4905619200000000001..44467660799999999999, _1416#=<_1346+ -1, _1416 in 4905619200000000000..4905705599999999999, 4905619200000000000+_1484#=_1416, _1484 in 0..86399999999999, _1306 in 0..86399999999999.