Hi Rande,
You need something like this; (substring-before(substring-after(field, "T"), ":") * 60) + (substring-before(substring-after(field, ":"), ":"))
Note: this assumes date and time. If it's just try this: substring-before(field, ":") * 60) + (substring-before(substring-after(field, ":"), ":"))
:)