I used lantus for 18 years and if I forgot a dose ( evening 6pm )
I would just cover during the next day during the day with extra correction doses of fast acting until 6pm arrived and take my normal dose.
( this only works if you are on an MDI regime -- multiple daily injections )
if you are only using the 1 insulin ( lantus) as a background along with oral meds I would personally just skip the dose and start again at your normal dose time this evening.
ok you may run a bit high today -- but trying to take a partial dose any earlier will just lead to a bit of stacking during the night tonight so I would leave it .
you can buy a device that tells you when you last took a dose of your insulin -- link here www.timesulin.com
Last week I had no recollection of injecting Lantus in the evening and it out me into a panic situation. I posted an emergency thread. As it was the morning reading was fine so I probably did.