The word “secular” can mean simply non-religious but really means more than that; according to the Online Etymological Dictionary, secular means
“worldly, pertaining to a generation or age,” from Latin saecularis “of an age, occurring once in an age,” from saeculum “age, span of time, generation.”
The basic distinction is between matters that pertain to the age and world in which we live and those matters which are beyond it — life after death, unseen spiritual reality, etc.
Modern science has always focused on the secular in this sense, and abstained from investigating metaphysical and spiritual matters — but that should include “deep time,” too. By definition “deep time” refers to ages of time before this present age, this age of human life. No human being ever lived in deep time. Human experience does not include deep time. A scientific organization (or a government) cannot promote belief in deep time and remain secular.
Secular science should exclude everything that is not part of the age and world in which humanity lives. That means secular science must remain within recorded history, the period of time covered by written sources. This may be extended slightly by the study of artifacts for societies without writing.
Since scientific creationism stays within recorded history, it is more secular than any deep time theory.